I've been in a similar boat as you, but I'll do you one better: I cheated one night after drinking too much, because my girlfriend said that she wanted to break up after we finished school in a month. It was easily the most selfish, hurtful, nasty thing I've ever done. I was hurt by what she was saying, but I hurt her more than I could have ever conceived.

So I (as you did) confessed and it ended, and I tried to forgive myself, but forgiveness never came. I, like you, considered myself a good person, so how could I do something so terrible?

I spent the better part of a year agonizing over what I had done, and I can see now, in hindsight, that that self-hatred and shame, while coming from a good place, was ultimately counterproductive and made a bad situation worse. I wasted time, I threw away opportunities, because I felt that I was scum and I didn't deserve to be happy. But the sun kept coming up and world still spun, until finally I looked around and realized that my life had become dedicated to making myself feel shitty, day in and day out, again and again, over something that had happened in the past and was gone. My hands twitched, I vomited every morning before work, I drank myself to death every weekend -- I was a total wreck.

So I let it go. I still didn't forgiven myself then, and I don't know if I ever will. But, at a certain point, I had to just move on and try to carry on my life as best I could. It was hard, sure, but the alternative was getting me nowhere.

You should feel bad for what you've done. You know how terrible it was and you don't need a lecture on morality. But, look, don't whip yourself too bad, or you may find yourself really believing the voice that says "I am a bad person." I don't think you are, if you were a bad person you wouldn't feel as bad as you clearly do.

My only advice going forward is make amends as best you can, acknowledge your misdeed candidly, and put it behind you as best you can. Doing good can make you feel good, so working for charity or even doing small favors for friends can go a long way to restoring your personal pride.

Bear in mind: Martin Luther King Jr., a quintessential "good person" in history, conducted numerous sexual liaisons with other women while he was married. We can and do acknowledge that this was a scummy thing for Dr. King to do, but we still remember him for the great work that he did, not his shame; we still admire him not because we ignore his flaws, but in spite of them. Good luck.