I'm Sorry
Sherlock did not know the true reason as to why Molly and Tom's engagement ended. He had his theories of course, but most of those theories revolved around Sherlock himself. Tom likely realized that he was a poor imitation for Molly being able to finally have a relationship with Sherlock himself. Or perhaps, after the wedding, Molly was finally able to see that Tom would never be able to live up to her intellectual needs of a relationship. That moron did come up with meat dagger after all. And there was always the idea that now that Sherlock had returned from his mission abroad that Molly had fallen back into her old ways; back into the mind of a pathologist who was infatuated with a high functioning sociopath.
However, it would come to light that all of these theories were wrong.
"Clean?"
The first slap takes him off guard. In all the years that he had known Molly Hooper, he had never known her to have such a fire. The following two are less of a surprise, but they still leave his face stinging with pain.
"How dare you throw away the beautiful gifts that you were born with? And how dare you betray the love of your friends? Say you're sorry!"
"Sorry you're engagement's over. Though, I'm fairly grateful for the lack of a ring."
Old Molly would have run away by now. Old Molly would not have had the backbone to stand up to Sherlock when he was being such a careless idiot. But she wasn't Old Molly. That woman was long gone. "Stop it. Just stop it."
It's close to four months later before Sherlock gets some insight into the breakdown of Molly's engagement. Solving the Moriarty situation that arose took time and all of his mind palace. Luckily he was able to wrap everything up a week before Mary gave birth to Emma Marie Watson. A small pink bundle that caused tears to be brought into John's eyes as he held his daughter and kissed his wife. When John passed Emma to Sherlock arms, after ensuring that he was properly supporting her, Sherlock mentally catalogued her features. His life was now about protecting her.
It was as he was filing the sound that the infant made when she was content that Molly walked into the room. She was still wearing her white lab coat, likely coming up to visit the new parent's on her break. She had a smile on her face, with congratulations for John and Mary on the tip of her tongue. But when she sees that pink skin, and tiny hands, and blue eyes, her breath gets caught in her chest.
Mary smiles knowingly at the pathologists reaction, "Molly, we would like you to meet Emma Marie Watson."
Molly can feel the salty solution of her tears form behind her eyes. She was so beautiful. "She's lovely."
John nudged Sherlock gently to pass his daughter over to Molly for her to hold. Molly was hesitant at first, but once Emma was pressed against her chest and cooing softly she was content in rocking the infant back and forth. Once Molly's arms grew tired, she passed the sleeping infant back over to her mother. Mary and John had a glow about them that only new parents could have. That image put an ache in Molly's heart, but she pushed it aside.
"Molly. Sherlock. Mary and I have something important that we would like to ask of you both. If anything were to happen to us, we would like you two to look after Emma; be her guardians."
Sherlock stood a bit straighter at John's statement. "You have my word. I vowed that whatever happens, I would always be there for you three. That will never change."
John smiled at his dear friend and passed a quick hug onto him, clapping his back a few times. He always knew that when it came down to it that he could count on his friend.
"Molly?"
Molly sucked in the breath that she was holding and pasted on a smile through her tears. She was truly touched by the thought that Mary and John trusted her with their daughter. But it still hurt. "Of course. Of course!"
On one of Sherlock's weekly visits to the Watson residence to see monitor how Emma is developing, Sherlock brings up a topic that had been on his mind.
"I didn't realize that you and Molly were so close."
Mary looked up from tickling Emma's stomach. "Quite beating around the bush, Sherlock."
"I was simply making an observation that-"
"I'm not John, Sherlock. Get to the point."
Sherlock sighed, clearly the internet theory of baby brain had not had an effect on the new mother. "I was… surprised at your decision to ask Molly to be Emma's guardian."
"Are you jealous?" Sherlock rolled his eyes at the blonde's comment. "To be honest, Molly and I got quite close after her engagement ended. I was sure about asking her to be Emma's guardian. John and I weren't sure if she would be ready with everything that was going on."
Sherlock didn't bother with a response and merely raised an eyebrow at the woman.
"I'm talking about her and Tom."
"What does meat dagger have to do with this? She ended their relationship nearly six months ago. I hardly think that she would be upset by her decision."
"Oh Sherlock, you really have no idea do you?" Mary placed Emma in her crib and turned on the monitor. "Tom broke things off with her after she got the news."
"What news? Molly didn't tell me about any news."
"Sherlock, I… If you don't know, then I can't tell you. That's not fair to Molly. I'm sorry."
Mary had been kind enough to phone Molly as soon as Sherlock left to warn her. She had apologized profusely for mentioning as much as she had. Mary had assumed that Molly would have either told Sherlock or that the detective would have deduced it by now. As brilliant as Sherlock was, he could be quite oblivious at times. Molly had assured her friend that everything was fine and that she would handle Sherlock once he barged into the lab (which she knew she was). The pathologist should have known better than to think that she could keep this to herself forever.
Sure enough, thirty minutes later Sherlock pushed his way through the double doors of the lab. "Ah, Molly! I didn't realise that you were working today. What a coincidence!"
"Mary has already called me, Sherlock. You can drop the act."
She knew that he wouldn't be able to resist asking her what was going on for long. Sherlock was like her cat Toby in many ways. Too curious for his own good.
As Molly spoke she continue to methodically do her work. It was the only way that she could keep her voice from shaking. "Just after the wedding, I had missed my period. I thought that I might be pregnant. Tom was thrilled with the idea, so I booked an appointment with my Doctor. The test came back negative. I had some more run. Turns out that I'm not able to have children."
Molly left a pause for Sherlock to deduce the rest on his own and state the obvious like he usually did. However his baritone voice did not fill the room like she expected it to.
"It also Turns out that not being able to have children was a deal breaker for him."
Sherlock did not know what to say. He knew that whatever he did was likely to be the wrong thing, as it so often was (socially speaking). He also knew that this woman that he had come to respect and care for was already hurting, and he didn't want to add to her pain. So he said the one thing that she told him to say all those months ago.
"I'm Sorry."