I did something bad.... how to get over it...?

I cheated and it was one of the reasons that contributed to the demise of my long term (7) year relationship. I cheated on someone I had feelings for since my childhood and it happened quite a few times. I don't talk to the childhood guy anymore and my long term partner doesn't want to reconcile. Just after our separation, my long term partner told me he also slept with someone (I also found him online on dating websites looking for sometime back). My relationship wasn't perfect, and I was unhappy since my partner would ignore me a lot but I cannot get over what I did. It wasn't all bad either (my relationship). The cheating happened 1.5 year ago and I still beat myself up every day for it. I go to therapy to deal with the guilt but nothing makes it go away. I have asked for forgiveness but my long term partner says he will never forgive me. I've prayed, I've written a letter, journalled, confessed, everything but the guilt just does not leave me. Is it normal to feel this bad for so long? I never want to cheat again. I got so swept up and thought I was in love with the childhood guy (he was so sweet to me) and lost everything i built up for those seven years with my long term partner who i adored and idealized. Help... How will I / will I ever get over the guilt? I cannot see or hear of anyone talking about cheating without feeling ike a total loser. Like I need to be wearing a scarlet letter on my chest.