How to forget

My boyfriend had a nonphysical sexual relationship with several women when we first started dating. It took a while before I found out and he was never entirely honest, most of the true details were ones I had discovered myself.

It's been over a year and I still think about it EVERY SINGLE day. I use to struggle with my weight but had finally reached a point where I loved the way I looked. I was confident and happy. Discovering his infidelity, seeing all of the pictures and videos, completely broke me.

We've worked on mending our relationship and things had gotten better. He recently mentioned a girl that he works with. Very normal things, normal conversation but I'm making myself sick with worry.

How can I forget the man he use to be? How can I focus on the present, the things I know and not compare every detail to things that happened in the past?