Question about the "other woman"?

I made the mistake of hooking up with a guy(certain phone talks multiple times/second base stuff only once) and I knew he had a gf. It was so out of character of me and I despise myself for letting him get to me and I somehow convinced myself that I was falling for him when really I was on the side. His girlfriend knows (about the whole thing and that it was me). And I don't really speak to him anymore. I know that I will never ever ever do something like this ever again because not only is it not fair to me but more importantly not fair to her because I am just as an idiot/stupid/disrespectful as her bf(who I don't understand why she is still with?). So basically my question is, that can I ever be redeemable after what I have done and should I just take this as lapse in judgment and use it as a learning experience?"Sorry for re post", I want to see how many people say the same thing as my friend( he said to never do this again, forgive myself for my mistake and move on with life and that I am still a good person, which I disagree with-in my opinion good people don't do what I did)