He admitted it and I'm at a loss

We've been together for 6 years and getting married in a couple months. He (24m) told me (24F) yesterday completely unprompted that he had sex with a coworker multiple times when we first started living together about 3-4 years ago. They stopped after 6 months he called it off because he didn't want to do that to me anymore and proposed a couple months later. If he wouldn't have told me I would have never known. He says it's eaten at him for a long time and he wanted to go into the marriage with everything on the table and no secrets if I decide to stay. He wants to be a better person for me and in general. I'm scared because he's lied a lot about things but never actually cheated on me emotionally or physically and that's the thing I always held close to me. He had no romantic involvement or crush or anything on her that I know for sure he just did it to "have fun". I'm absolutely heartbroken. Is it worth it to stay and rebuild? All those memories while he was doing that hurt so bad. I'm just at a loss and I don't want to lose him.