I believe that everything happens for a reason. People change so that you can learn to let go, things go wrong so that you appreciate them when they're right, you believe lies so you eventually learn to trust no one but yourself, and sometimes good things fall apart so better things can fall together.
No matter what choice you make, it doesn't define you. Not forever. People can make bad choices and change their minds and hearts and do good things later; just as people can make good choices and then turn around and walk a bad path. No choice we make lasts our whole life. If there's ever a choice you've made that you no longer agree with, you can make another choice.
It's funny about love', Sophia said. 'The more you love someone, the less he likes you back.'
'That's very true,' Grandmother observed. 'And so what do you do?'
'You go on loving,' said Sophia threateningly. 'You love harder and harder.
— Yoko Ono
You can spend minutes, hours, days, weeks, or even months over-analyzing a situation; trying to put the pieces together, justifying what could've, would've happened... or you can just leave the pieces on the floor and move the fuck on.
You may not be her first, her last, or her only. She loved before she may love again. But if she loves you now, what else matters? She's not perfect—you aren't either, and the two of you may never be perfect together but if she can make you laugh, cause you to think twice, and admit to being human and making mistakes, hold onto her and give her the most you can. She may not be thinking about you every second of the day, but she will give you a part of her that she knows you can break—her heart. So don't hurt her, don't change her, don't analyze and don't expect more than she can give. Smile when she makes you happy, let her know when she makes you mad, and miss her when she's not there.