I cheated

I'm a straight guy, I have an amazing girlfriend. A few months ago I cheated on her (from her point of view). I was still getting over my porn addiction, 3 months clean. One day I relapsed and felt horrible about it. It took me almost a week to get the guts to tell her about it, and she was really upset. She was so very generous and forgave me, but I still feel horrible about it and it really messes with my depression. How do I forgive myself?