I hooked up with my best friend's son. The guilt is eating me alive. Should I tell her?

Anonymous
I have known him since 2011 and he was a little boy. He is 21 and I am 39. He has been attracted to me for a long time and I recently started being attracted to him now that he is getting older. He would come around more and more when his mom and I would hang out together and drink. The three of us would talk about our bad relationships and one day out of the blue he messaged me on Facebook and told me he had feelings for me and was attracted to me. I was in the process of breaking up with my ex fiance and his attention to me allowed me to finally go through with it. He would text me from 6:30 am when he got to work until 10:30 pm when his girlfriend got home from work. On the weekends I wouldn't hear from him because he would be with his girlfriend. After a couple weeks of texting we finally hooked up at my house and we did that several times. Then his girlfriend decided to start going through his phone so he called it off at the end of July.

I agreed to hook up with him because he always told me he would leave her when his lease ends. Although that could be true, it doesn't seem that is going to happen. I feel used and guilty. It's hard for me to see his mom and hear her talk about him and his girlfriend as if they are solid and nothing happened. It is hard for me to see his mom and pretend that nothing between he and I happened. I feel bad for the girlfriend because he has misled me and is currently misleading her. I feel like a piece of shit for what I have done and allowing a kid to talk me into something that I knew was wrong but I was feeling vulnerable at the time and he made me feel wanted and desired.

Now I am having nightmares every single night about what I have done and how I betrayed his mom and his girlfriend (who I have met twice). He is avoiding me and I have not seen him since the last time we hooked up on July 19th and I have not talked to him since Aug 1. Do I come clean to alleviate my guilt or just keep my mouth shut?
Updates
+1 y
I wanted to let some of the comments roll in before I addressed questions. I am not a mother myself so that is how I could separate how my friend would feel to an extent. People assume I am in my mid twenties because I look very young. As far as his relationship, he is currently with a girl he has known since they were babies and although he loves her he is not in love with her. He stays with her for security and because he doesn't want to hurt her. They share a dog and a rental together.
I hooked up with my best friend's son. The guilt is eating me alive. Should I tell her?
112 Opinion