Years ago I read the book "Bringing Up Boys" by Dr. Dobson. Not related anyway to porn or masturbation, but because I have two sons and I want to be a good Dad. In reading I was shocked to read that Dr. Dobson didn't say that masturbation was sin in and of itself, that it was quite natural for adolescence to do. Now I don't think you're still discovering your body so perhaps that doesn't apply to you. In the broadest strokes, sin is when we chose not to follow Him. Which can be something as benign as an extra spoonful of ice cream or gossip or as horrifying as stealing or infidelity. What I am trying to do is give you permission to forgive yourself. I heard in a sermon recently that convicting thoughts come from God but condemning thoughts come from somewhere else--be convicted and move forward, but don't let the spirit of condemnation have any thought space--you are good enough and God knows you and Loves you anyways.
Strongholds are tough to shake, hence the name. Prayers bro.