A/N: Spoilers for "Heir of the Demon" and I guess series 3 finale of Doctor Who if you haven't watched it.
It didn't happen quickly. No, Felicity came to the conclusion, slept on it (a few times), and then finally resolved herself to take action.
A full week of discomfort and hurt later, Felicity knew what she had to do.
She waited until Sara, Roy, and Diggle had left for the night (or rather morning) and she had Oliver to herself.
They had hardly spoken in the last seven days, other than discussing work-related details of either job. In fact, Oliver could hardly meet her eye. His mother's words were the soundtrack to Felicity's waking nightmare. She couldn't help but wonder if this would've happened if she'd just kept her mouth shut about Thea.
But regrets weren't going to get her anywhere. It was time to make a choice and for once, Felicity was choosing herself.
"After Russia," she began quietly, seeing him start a little at the sound of her voice, "you said that you couldn't be with someone you really cared for." She met his gaze with an uncertain smile that she could feel shaking at edges. Oh God, she thought, don't let me cry. Please just let me get through this with my pride. "But what you really meant - and what I'm just now understanding - is that you could. It just had to be someone for whom you felt strongly enough to take that risk. And obviously that wasn't me."
"Felicity," he murmured, but she held up a hand to stop him."
"Oliver, we both know there has always been something more than friendship or a partnership between us. At first, I thought it was unrequited, then it seemed like it wasn't. And now? Now I don't know what to think, because you're telling me we can't in one breath but then giving me hope for more with the next." She shook her head in frustration. "Until this last week."
Oliver took a step towards her. "What Sara and I have is-"
She stopped him again. "It's complicated. And historic. And so so so messy. I get it. And I like Sara and if she makes you happy, then I'm happy for both of you. But, Oliver, this - us - it's got to stop."
He shook his head and tried to reach for her arm. "Felicity," he said in his low, emotional voice and for a moment her resolved faltered.
Stepping back, out of his reach, she smiled sadly. "You know how I love Doctor Who? The first few seasons of the new series had this great heroine. Rose. She was amazing and smart and she and the Doctor had this epic, beautiful love story. And then they were parted tragically. After Rose, the Doctor meets the equally amazing Martha Jones. And she falls for him, immediately, because who wouldn't when this handsome, secretive man drops into your life and whisks you into a life of adventure?" Her smile turns a bit brighter for a moment, then drops.
"But in the end, Martha realizes no matter how much she helps the Doctor or what she sacrifices for him or how desperately she wants his love, she's not Rose. She will never be able to fill that slot in his life. So in the end, Martha Jones stands in front of this amazing man and tells him, this is me, getting out. And she chooses herself and she finds her own ways to help fight the Doctor's battles. And she drops in on a few more adventures. But Martha chooses herself."
Oliver's face has lost all color by this point. His mouth is a stern bracket, his jaw clenched tightly.
Felicity grabs her purse and an extra bag full of personal belongings she's left around and drapes the straps of both over her shoulder. Taking a step towards Oliver, she puts her hand over one of his clenched fists and waits for him to look her in the eye.
"Oliver, this is me, getting out."