I can relate a lot to what you're going through. When I first started masturbating I felt guilty about it afterwards, which was a result of my religious upbringing and thinking it was a sin. I would pray forgiveness after I did it then go a bit without doing it only to fall back into it. I thought I had an addiction. It wasn't until I got up the courage to tell my mom about it that I learned it was okay and I had nothing to be ashamed of and then I didn't feel guilty anymore.

I also through the years discovered that even though I don't believe those same beliefs as strongly as I once did, my spirituality still influence a lot of my line of thinking subconsciously and even my creative juices. Have you ruled out the possibility of your religious upbringing giving you subconscious guilt? It's probably not the case if you haven't felt guilt for premarital sex but I thought I'd ask anyway. I don't know a whole lot about Catholicism but from what little I do know they do seem to harp on masturbation a lot.