personal responsibility

Why can't I let go of his past?

I am 21 years old and have been with my boyfriend for eight months. This is the first relationship I have ever been in; we had sex only a couple of days after we met and I have been with him since. Obviously I trust him and love him and we have a very healthy relationship, but I have known for a while now that prior to meeting me, he slept with 15 other girls....