Actually, she already knows the answer, because she's screwed up enough opportunities already.
The first chance she had to make the right choice for him, she did. She said no, and stayed in that alley, arms around his pathetic lump, turning down the Doctor's promises, all for him. He needed her, and she knew that, and after what he'd seen it just wouldn't be fair to leave him alone.
The second chance came not a minute later, and she blew it. Travelling in time was suddenly way higher on her list of priorities than taking care of her boyfriend - after all, weren't men supposed to be the protectors? Not that she was a particularly traditional sort of girl, but it wasn't asking much to expect him to at least pretend to want to help, at least pretend that he wasn't scared out of his wits. Was it? And instead she could be visiting other planets, other millennia, with a man who'd not only tried to keep her safe, but who'd been trying to save the world. A man whose life she'd already helped save, proving to herself that she was no Mickey Smith. There was just no competition. Off she went. Strike one.
The third chance was the Doctor's fault. She wasn't in charge of flying the TARDIS, so when she landed at the Powell Estate a year after leaving, she felt guilty but not responsible. And okay, it wouldn't have happened at all if she'd stayed, but after everything she'd seen she couldn't regret that. Strike two. She briefly considered not getting back on the TARDIS, but then the spaceship crashed into Big Ben and it was all the adrenalin, all the excitement that she'd become addicted to, and she knew she couldn't give that up yet. She couldn't help but be proud of Mickey when he proved he had the guts to blow up Downing Street, and she was a little disappointed when the Doctor wouldn't let him on board, but she had to admit there was a part of her that just wanted it all to herself for a bit longer. But she'd asked, and that made a difference - she felt that she hadn't blown that one. The fourth chance.
The fifth chance wasn't one that Mickey needed to know about. Realising that despite wanting it all for herself, she actually missed him a little, missed the casual flirting and the company of someone who clearly found her attractive, she'd jumped at the opportunity to have another man on board. The Doctor was fantastic, but he didn't seem interested in a relationship, and certainly didn't respond to her quite how she'd like. So Adam seemed like a good idea. Nothing actually happened - even if it might have, he quickly proved that he wasn't worth the trouble he caused and was taken home. But in her mind, asking someone else along was undoubtedly another betrayal. Strike three.
He'd never remember the sixth one, where he'd clung to her as a scared little boy, somehow recognising who she was despite the fact that in his timeline she was just a baby. That time, she was there for him.
The seventh chance, strike four, echoed the fifth in some ways, and yet at the same time it was so very different. She was swept off her feet by Jack, and she realised now that even if he'd just been someone she'd met in a bar there'd've been major friction with Mickey. Instead, he was now travelling the universe with her, just her and the strange Doctor who might well 'dance' but seemed determined not to do it with her - waltzing around the control room aside of course. And when Mickey walked in that door this morning... she'd seen the look on his face. The presence of Jumping Jack Flash did not impress him one bit. Even when she'd told him that actually she didn't need her passport at all, letting him know that she'd just wanted to see him, something just wasn't quite right. And somehow, when he told her that he was going out with Trisha Delaney, she was surprised but not shocked. Of course, that hadn't stopped her rounding on him, feeling like he'd let her down.
Actually, she realises now, she's always been the one who does that. And it hurts to realise that she's lost something today, and that it's entirely her own fault. She might blame the Doctor, she might even blame Mickey for being a pathetic lump who can't even guard a door without getting it wrong, but she knows that's not fair. She chose him once, and they were happy; it wasn't an exhilarating ecstatic sort of happiness, but they'd been content. She understands that for her it's not enough anymore, not now she's seen so much, been so many places. Mickey's never done anything wrong; he's just sat and waited for her. But it's not what she wants. And she knows that she doesn't care enough to stay.
She's sorry. But she's just not sorry enough to give this up.