Cheating

I was wondering how you overcome you S/O cheating on you. 2 years ago I cheated on him fast forward to no he proposed to me the the other day I find out he has been cheating on me with our roommate/god children’s mother/my best friend..... she says it’s been going on for 2 years he said 2 times.... I am willing to work on our relationship because he gave me a chance when I cheated so it’s only fair I kicked her out and made him delete her or so he says... I just want to act like nothing happened but how? How do you over come this sort of thing?