Come clean or keep dirty secrets?

My SO and I broke up last year and I began talking to another guy who was interested in me though I wasn't interested in him..  he came on too strong and would touch/kiss me even though i didnt want to. Anyway my SO and I are trying to fix things but I found out he's friends with the guy I was talking to. I'm scared the guy will ruin my relationship.  I don't know what to do.  Should I tell my SO? I'm so scared of losing him!!! 😢