“I had sex and want to come clean”

Last year, I had sex with a guy, only to find out that he had a girlfriend. I honestly felt terrible because he continued to go on dates with her since, and she had no idea what happened. I recently found her social media and saw that she had posted a picture of them together.

I really want to come clean and tell her what happened between her boyfriend and me, but I am scared of what is going to happen next. I need advice on whether or not I should tell her, and I also need advice on what I would say if I told her.