Should I tell him

I've been with my S/O for 5 years now. He's my fist ever relationship and I haven't told him my biggest darkest secret yet. When I was 14-15 my cousins ( that I'm very close to dispite our 15 year difference) bf at the time started making passes at me (he was 27) and it developed into a relationship that involved everything sexual besides intercourse. I knew it was wrong but didn't know how to say no or stop it. He pretty much conditioned me for the following 4 and half years telling me who iam, what I should do with my life, how I should go and date more, how he loves me etc and I ate it up like candy. I thought I loved him but looking back it wasn't love at all. When I turned 18 I moved in with him and my cousin and we ended up having sex whenever we can until I met my current bf. I stopped having any kind of sexual relationship with my cousins bf and stopped talking to him. My S/O even knows him now and hung out with him lots of times.  I've been trying to tell my S/O 4 times already in the past 6 months but eveytime he doesn't want to hear it or says if it didn't happen while we were in a relationship together then keep it to yourself. But it's been eating me up inside. Should I tell him no matter what or should I just keep it to myself?