I just found out my boyfriend of six years cheated on me 4 years ago, what do I do?

I have been with my boyfriend for 6 years now and have always suspected that he cheated on me multiple times the first few years we were together. Of course I never had proof so I stayed with him. Well I finally have my proof and I don't know what to do since it was 4 years ago and we are doing a lot better now than we used to. I found out he cheated on me with a girl he worked with and he told me in so many words that he quit seeing her because she moved away. He also keeps changing his story of when it happened precisely and never would have admitted it to me had he not gotten caught. What do I do? Stay since it was a long time ago or leave?
I just found out my boyfriend of six years cheated on me 4 years ago, what do I do?
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