I know a nice girl with sexual past, she is 15 and had sex with multiple guys, with only one in relationship. So I find this not nice, she said sorry and everything, and I've tried to forget but i just can't, please help me?
I dont think you can. The people who accept their partners past are able to do so because it never bothered them that much in the first place. If it bothers you a lot I dont think you'll be able to truly accept it.
Most people dont end up getting married to the person they dated at 15 anyways so maybe just break up and look for someone else.