I treated a wonderful man really horribly. He has forgiven me and moved on. I'm happy for him. Everyday I can't stop thinking about all that I did and how he must have felt. I regret it and hate myself so much. How can I ever forgive myself?
You still feel guilty because only "feeling guilty" does not undo what you did!
You have to seek redemption in action, not feelings. Feelings and wishes are no more than theory. Your need practice!
Maybe it's not possible for you to undo your bad dids to that one man, but you can do good to other people in general and particularly to some other man that comes to take the very same place the first one took.
Stop standing still by guilt and start taking real action. Doing so, peace will warmly kiss you and be your forever companion.