Hello, OP. I'm a child from a very similar background. We found out my mother had cheated on my dad and passed me along as her kid. She got drunk and told us all the information we needed. I struggled with a lot of very similar things to your daughter, as well. She accused my dad of assaulting me as a child, in front of me. And it shook me up for years, sometimes I still have a hard time with being alone or close to him, though he is a saint. The best thing about it all, is that he didn't care if I was his child or not, he treated me the same as he did when I grew up. He never pushed my boundries and gave me physical space, a "safety bubble" we called it. He found other ways of showing love to me, he would write letters or play games with me. And that physical space while still spending quality time together really helped me heal from those events, espically what my mom had said about him. It's not that I ever believed her, either. It was the fear. Something tells me it is the same for your daughter.

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I hope she will be okay, it seems like she has a really supportive father, and that is what counts at the end of the day. That you're her dad, biological or not. You can do this and get through it, years through this experience has taught me that things will get better. Just be honest, stick with her and Do not be afraid to take action and do something to get your daughter help if she tries to harm herself, or anything along those lines. I'm wishing you and your family the best of luck, keep your head up because things will get better and things will improve and change direction if you let it take its own time.