He cheated on his girlfriend

I went to a friend’s party took a few sh*ts on an empty stomach and got tip$y fast. That night became my biggest mistake.

I arrived around 7:30 pm caught up with old friends then suddenly saw my ex. She came over we talked and I found out she hasn’t dat*d anyone since we broke up.

Later we all played truth or dare. They dared us to go into a room alone. We ended up talking for hours. She locked the door, suggested we keep playing truth or dare just the two of us. Things got w*ld quickly we ended up nak*d, then we h**ked up. It was int*nse and l*ud. We went for about two hours. Afterwards, she said she has a boyfriend I told her I have a girlfriend too. We both sh*ck*d. We agreed to keep it secret and hope nobody talks. It’s been two days no one has said anything yet. But I’ve already texted her asking to meet again.

Now I regret it deeply. The guilt is killing me. I don’t know if I can hide this from my girlfriend forever. Should I tell her the truth or keep quiet and pray it stays buried? What do you think?