How do you forgive amd forget?

So my boyfriend and I had been together 3 years.We got in a big argument and I found out the next morning he had gone to see a girl and had been seeing her in another state almost constantly...She found my name and called me told me they had had sex and fast farward we decided to get better and work on us...so far I absolutely love being around him I get so happy and all my pain just goes away I feel so safe and cry so hard every time he has to leave.I do love him very very much but when I'm not around him I think about what he did and the fact that he had sex with her...me and him lost our virginites to eachother so we were eachothers one and only...I cry all day all night I don't want to eat I always feel sick but when I'm around him I'm always so happy...advice?