Help please. Need advise

So I feel really bad for something I did before me and my bf broke up a year ago for 3 months. I knew our relationship was going to shit and I did something nothing bad and we broke up and I thought we were never going to be together again. We were together for 2 1/2 years then and broke up for 3 months. He hurt me a lot on the past and that's what pushed me to make a big mistake which I felt horrible about right after but kept it a secret because I knew we were gonna break up anyway. We were both in a bad spot. We got back together and a year and a half later I feel so god damn bad about it now. Our relationship grew and were on a whole new level. Our future has never been more pure or clear. I can't ever tell him about that mistake, I'm too scared to lose him and I even forget about it sometimes.. we broke up so when we got back together I saw it as a renewal and a fresh start so I prolonged it more but now I'm in a spot where I'm feeling guilty and bad, but also I'm feeling like we broke up so it shouldn't matter now because I never did anything or will ever do something like that again. Give me some advise. (I wish I could tell him and I would if it wasn't so fucking horrible. It makes me sick to my stomach) I feel like a new person now and I know it would crush him.. I don't blame him but I can't hurt him like that. I never want to hurt him, he's been through so much already. I was careless and stupid before and we both changed this past 2 years.. idk what to do it makes me want to cry. When I think about it, and think about telling him, it's almost like I'd be lying to him. I still cant believe I went off and did something like that.. I'm so disgusted with myself and I just wish so god damn bad that it didn't happen. I wish I could take it back so bad.