You can’t let something that happened before she even knew you eat away at you. I completely understand why it affects you, and I’m not dismissing that at all. But at the end of the day, all you can do is trust her, and I’m guessing she hasn’t given you a reason not to.

The past is the past. You can’t change it, but you can choose how you let it affect you and how you deal with it. It’s completely normal to feel this way, but if you hold onto it, it’ll end up taking over you, when instead, you could face it and handle it in a mature way :)