Secrets uncovered and I don't know what to do.

This year has been hell, and yesterday it only got worse.

My fiancé and I have been together since I was 15 and he was 16. We had lost our virginities to each other, or so I thought, and have been through everything imaginable. Miscarriage, addiction, recovery, and so on. The other day I got word from my maid of honor, who used to be best friends with girl in question, that he may not have been a virgin. I didn't think much of it because the day before we had shared stories about the first times we ever had sex and in that she had heard I said we were both virgins didn't know what we were doing and never said anything. During the time she had said this my fiancé's mother was around and had said she didn't like the girl and had seen messages so it got me thinking. I had brought it up in front of him and he looked away and had said he had been wanting to tell me this for awhile. That he didn't consider it his first time because he was 13, she was older, and it didn't mean anything to him/didn't want to. That it had started and ended quickly, with neither of them finishing having sex because they were around other people and had never been back with each other. Let me be clear and say I'd never would have had an issue had he told me when we first started dating. Also, that I never asked him if he was, he told me he was and then asked if I had any other previous partners. Now it's been 7 years and I can't look at anything in our relationship the same. I can't even go through out the day without thinking about all of it. It's literally eating me alive.

I just needed to vent and maybe someone could give me their opinion if I'm over reacting or what should be done.