Should I say stmg or keep my mouth shut?

Hey guys! I need advice I think I know what I should do but I'm not sure. So I messed up and I had sex with my ex. Later that day I felt really bad and I really regret it. I don't know if I was just horny because I haven't had sex in so long. I was trying to be celibate. Well y'all know how well that worked out. Smh! He does have a girlfriend and she's the girl he got with after we broke up. We do know each other cause of him because he did cheat on me with her and he called it off with her n he was with me n he lied of course saying he's not talking to her anymore. Drama Drama!! To make a long story short! Do I tell her or should I leave it alone ?? I can't really talk to anyone about this that I know cuz I don't feel like getting judged. I called everything off I'm ignoring him. Help guys! Honestly I feel like she deserves this karma cuz she cheated on her husband with my ex. No lie I do want to throw it in her face. I promise I'm not that evil. I'm really sweet. Just fell for a fuck boy!! Smdh