Forgive and forget?

I’ve been dating my bf for over 4 years. When we started dating we were 17. When we first started hanging out I had my suspicions that he was seeing someone else. He had a girl “best friend” that he spent a lot of time with and who posted pictures of them looking close. He said they had known eachother since childhood. When I brought up my suspicions he consistently insisted that they were just friends. I was young and naive and I believed him but deep down I always knew there was something there. When we made it official I never heard about her again. He unfollowed her on all social media- weird considering they were “just” friends. I gave him some opportunities to tell me the truth and he insisted that they were always just friends. I tried to forget about it as throughout our relationship he’s never given me reason not to trust him and he makes me so happy. Last night he told me how serious he was about me and wanted to consider moving in together. I asked him about what had truly happened the summer we met and he confessed that there had been a 2 week overlap of him beginning to see me and dating her. He profusely apologized and told me how bad he felt for hurting her and me in the process.

I don’t know what to do. Deep down I have known this truth our entire relationship and I have honestly come to forgive him over the years but hearing him admit it makes it feel different somehow.