The past should stay in the past right?

So me and my boyfriend have been together for six years. We started dating when i was 14, being young and dumb back then i made a mistake that i didnt think would haunt me for the next six years. Everytime me and my partner get into a fight he always brings it up, no matter what. He doesnt see that it doesnt only hurt him anymore but its hurting me aswell. I just dont know what to do anymore. Yes everyone i cheated on my boyfriend, but it was six years ago. Ive showed him nothing but trust over the time we have been together.