Hurt, but Restoring !

Crazy question for you momma's but here's a little back ground. I've been with my SO for almost seven years, we got married in April and after we were married a huge secret came out. He slept with his bestfriend a few years ago. Mind you we were each other's first, and we have a baby girl already. Well his bestfriend he no longer communicates with her but she's still around due to his sister and the family . How do you handle getting over it? It hurts due I began to trust her and get close to her myself, I say I have forgive them but it's always in the back of my mind .. I overthink everything and we planned our second baby of course on good terms, but I need to get over the past just need help... or ideas ... those who have been through the hard times or have been hurt let me know ..