It's never been easy to accept someone's past, no matter how messed up it was. And finding that person who would wholeheartedly accept your every being, past, present, and who could you become, to really accept it without having to bring it up on every argument, is just as difficult as letting go of the past itself. Even to accept your own past, sometimes you need to break your back, let alone someone else's.
The past is always going to be a chapter in your life that you could never get rid of. There's always going to be complexity, catastrophic events, joyful moments in those chapters of your life, but then, it doesn't mean it can define your whole existence as a human. Your past does not define you.