Kara really didn't want to go to the wedding. She'd never felt so alone and clueless about the future. Mon-El was back in her life, and it hurt like hell knowing that he had a wife, but there was one thing that she knew would be worse. Losing Mon-El was bad enough, but watching Barry Allen get married to the love of his life… That would truly crush her.
She knew from the moment he swooped in and caught her from falling out the window of the CatCo building that there was something special about Barry. He had a knack for being so hopeful and optimistic; he never gave up, and that made him a real, genuine hero in her eyes. Every time she was around him, she could practically feel hope emanating off of him, this dogged determination that things would always work out in the end, and couldn't help but find that so appealing about him.
As cliche as she thought it was to admit to herself, Barry changed her life in the short time he was on her Earth. She thought Jimmy was someone special, but then came along Barry, and suddenly… Everything was different. She remembered kissing Jimmy after the Flash helped her defeat Silver Banshee and Livewire. Even if he was being brainwashed by Non, she could just tell from the kiss that her and Jimmy weren't meant to be, because instead of thinking about the man in front of her, Barry Allen was the only person she could think about.
She tried to shake it off, tried to convince herself that Barry was only a little crush, that she would easily get over him, but she couldn't do it. Even when she tried to get things to work with Jimmy, Kara just knew that it wouldn't work out. They both agreed that they just weren't compatible enough, and she didn't want to be unfair to him. Sure there was something between them, but it was nothing like what Barry made her feel.
Barry was on her mind a lot for months, and for a while, that stumped her. As if things weren't complicated enough in her love life, Kara soon faced another challenge: Mon-El, who was completely different from Barry. At first she thought the Daxamite was a self-entitled playboy, lacking in any admirable traits, but over time, he began to surprise her.
At first she thought he was nothing more than a partying womanizer, but she was mistaken. Mon-El was willing to change himself for her, to be a hero because of her, and she found that flattering. It was when Mon-El kissed her right before falling into a coma that she thought she was finally getting a break, that maybe she had a chance with someone who wasn't from a completely different world. But as luck would have it, that very same day was when she'd see Barry Allen again.
When Kara least expected it, there he was in her apartment. Just like that, all thoughts of Mon-El were gone, and she felt that inexplicable yearning that only Barry could make her feel. Smiling in his presence wasn't hard, but pulling away from the embrace she gave him was. He asked for her help to fight aliens threatening to his invade his Earth, and she was willing to join him right away. She didn't need to think about it. She would always be there to help him.
Being with him again, even just platonically, felt so strange and unreal. She never thought she'd see him again. Just when she started to have feelings for Mon-El, Barry Allen came running back into her life. It was like fate was messing with her, trying to confuse her, and it worked.
Following the battle with the Dominators, Kara came back to her Earth with her infatuation with Barry stronger than ever, and this time, her feelings left her so much more conflicted. He was in a relationship with a woman named Iris West, who he'd been in love with since they were children. She couldn't compete with that, and she definitely didn't have the right to. She could tell they loved each other, so she pushed her feelings aside. If Barry was happy with Iris, then Kara could be happy too right? She had Mon-El didn't she?
As sweet as Mon-El was, he was no Barry Allen to her. She felt horrible even thinking that, because she genuinely did care about Mon-El, but if it ever came down to her choosing him or Barry, Kara would choose Barry every single time. There was nothing she wouldn't do for Barry, and that scared her.
When she found out Mon-El was lying to her, she felt a lot emotions, but she felt stupid more than anything. How could he even think she wouldn't find out? She almost wanted his parents to take him off planet so that she wouldn't need to be so angry at him, and that he would be out of her life. Kara couldn't help but think that she should've known better than to trust a Daxamite.
Of course that wasn't the end of her and Mon-El, or her and Barry. Music Meister brought her back to Barry Allen in a bizarro musical alternate reality, one that she wished she could have lived in. It was such a nice break from the heartbreak she'd dealt with at the time; just singing songs with a man she was utterly infatuated with was just so corny and fun, she couldn't help but actually like it.
When she saw Iris and Mon-El together in the musical, she was hurt, sure, and a bit weirded out, but it reminded her of the way things should be; Barry with Iris and her with Mon-El. That way, no one would get hurt. No one but her. But she was used to shouldering problems for everyone else. Music Meister said he wanted to help two people with broken hearts, but hers was still fractured.
Kara left Barry's Earth with her relationship with Mon-El restored, and so she decided to make the best of things. Mon-El loved her, she knew, and she did love him too to some extent. She felt like she owed him a chance, and she didn't want to hurt him just because she was hurting herself.
As she opened up more to Mon-El, Kara began to see someone she could be with. She realized that if she kept comparing him to Barry, Mon-El wouldn't be right for her, so she didn't, and that made all the difference. Her feelings for Mon-El when she wasn't comparing him to Barry were so much more than she could have hoped. He was sweet, funny, and dorky in a way that worked (she ignored the fact that these were the same qualities she saw in Barry).
As luck would would have it, Mon-El, too, was yanked away from her. She made the air toxic to him, and she was forced to send him away in a rocket; she found it almost sadly poetic that she'd helped send Barry away the first time he left her Earth, and now she was sending Mon-El away in a rocket. History repeats itself, she figured.
Fast forward several months later, and there she was, no longer knowing who or what she wanted. She couldn't help but feel so hopeless, seeing Mon-El married, and especially because Barry was about to be married. Kara knew that moping over lost love and distancing herself from others would only end up hurting her more and more over time, but at the same time, love wasn't an easy thing for her to let go of. She was tired of being heartbroken.
Kara only agreed to go to the wedding because Alex convinced her. They both needed to get over their pain, and Kara wanted to be there for her sister, who was still upset with the breakup from Maggie, as well as for Barry, even if it was just as his friend. Kara knew that she was going to the wedding for people besides herself, and maybe that wasn't the best idea. She knew the wedding would only be salt in her wounds, but if there was one thing being Supergirl did to change her, it was instilling in her a habit to put others' needs before her own.
She returned to Barry's Earth with Alex, and she saw for herself how much in love Iris was with him. From the way Iris spoke of him, Kara could tell that they were meant to be. At the rehearsal dinner, she saw how everyone knew Barry and Iris were a match made in heaven, and it stung more than it should have. When she tried to get away from everyone by sneaking away to the balcony by herself, he found her.
Of course, Barry was wondering where her plus one was, and that only made her feel worse, if that was possible. She told him about Mon-El, about how he was married to someone else, and how being Supergirl should always be her main goal. She knew Barry would have told her that love was important, and she didn't want to hear it, from him of all people. She knew she was being really pessimistic, but she just couldn't deal with love anymore. She changed the topic, asking Barry what favor he wanted to ask of her. He wanted her to sing at the wedding and of course she agreed.
The day of the wedding came, and Kara woke up with a feeling of dread in her stomach. This was it—after today, Barry would be bonded to Iris forever. This wasn't how she wanted things to be, but Kara knew that it was how things should be. They made each other happy, and Kara knew that Barry's happiness mattered to her more than her own. She could move on, right?
As she sang Running Home to You in front of all Barry's friends and family, saw Iris walking up the aisle, and Barry crying, she felt a pang in her chest, she held back tears. She wanted things to be different, wanted so desperately for this all to be over. She wanted to be away from here. She focused on the song, but she couldn't help but realize how true the words she sang were for her. She wanted to have someone to run to, to help her mend what heartbreak she was feeling… She wanted him more than anything, and she was afraid to let go of him and how she felt.
As she sat next to Alex, listening to the minister speak to everyone in attendance, Kara felt like she was close to breaking. She thought she was going to be sick. Then the minister spoke the words, "Does anyone have just cause for why these two should not be married? Speak now, or forever hold your piece."
It all happened so fast—she knew that she wanted to tell Barry how she felt, but this wasn't the place for it. It was too late for her to tell him, but she stood up anyways. She couldn't just bury whatever feelings she had for him. "I have something to say," Kara said. She could feel their eyes on her—everyone's eyes—and she could feel shame and embarrassment as she tried to decide if she really wanted to admit her feelings in front of everyone.
"Barry, I—" she started, but that was all she could say, as a burst of plasma shot towards the minister, disintegrating him. She turned around, and saw her day had just gotten more hectic, as if that was even possible. Standing in the doorway to the church were…Nazis?
A little something I've had in mind for a while. May become a chaptered story if y'all want!