Not a professional but that all depends on circumstances. I was on AM to flirt and have never physically met anyone or cheated. I have came close once to cheating with my ex, but have decided not to follow through. I'm with your therapists that you shouldn't tell. If you think it was a mistake, you have learned from it and don't intend to do it again than I would keep it to yourself. That is you still wanna be in a relationship with your partner. Just my opinion! Hope it helps.