I'm ready to change.

I remember first looking at porn when I was in middle school. My parents did not go out of their way to give me a sex ed talk, so I, an afraid, insecure middle schooler, did not want to go ask them about it. So I asked the internet instead. There I was for the first time exposed to the world of porn.
After a few months I got off of it, went cold turkey on porn. I relapsed a time or two along the way, but overall, I was off of porn. However, while the porn disappeared from my life, the images stayed in my head, and so I had fuel to regularly jerk off to. This has been my major struggle. I have porn inside my head, and it won't go away.
I want to be done with this. I have an amazing girlfriend, and jerking off to fantasies of other girls makes me feel like I'm cheating on her. Also, as a Catholic, we regard masturbation as a grave sin. Lastly, I just want this for myself. I'm tired of jerking off when I have a bad day or I'm stressed or I'm lonely. I hope that through this I can move on. If there is anyone I can be accountable to, I'd greatly appreciate any help I can get.