I have really struggled with this in the past few years. I seem to always have terrible images and body memories when having sex with someone. Usually I seem to dissociate at the beginning to ward off any unpleasant sensations, just completely take myself out of the situation before it goes downhill. However, I've been working really hard to do things to stay present with my partner and enjoy the sex life we share. It doesn't always work though, and I do sometimes need to stop. Hopefully, the person you're with can share in those moments with you on an empathetic way, listening and comforting you instead of making you feel isolated and abnormal.