Help mind management during reboot.

Hi at all!
I need your advertisement on how to manage thoughts during reboot.
I'll try to shortly (definitely not something I'm good at) recap my situation, so you can better understand my problem and maybe point me a solution.
I'm 22, from Italy, and I started trying to quit porn and masturbation almost 3 years ago. There was a plenty of different reasons for this choice, some of them nowadays doesn't hold anymore, but still I find P&M to cause me mood swing, before and after, damaging my normal life activity. I'm in a very nice relationship and we have sex regularly, let's say 2/3 times a week.
In these years I tried a number of times to totally quit PM, and at the moment I've reached a sort of balance, that is I feel need to fap once a week (before trying to stop, was almost daily). Anyway, I meet lot of resistence to push forward. After 2 weeks usually (I overcame it 8 times) I begin thinking at porn, initially are short flash, images, that appears in my mind and that I can easily remove, but gradually they become more and more insisting, stronger and harder to remove. From about 3 weeks on, this begins to affect my quotidian life, make harder to sleep at night and to focus during day, and inevitably, when I start to see my study, my sports performances and my relationship drop because of these obsessing thoughts, I relapse and restart with usual rythm.
My question is, how can I overcome this phases (cause I'm sure after some point these images will start to fade out) without having all others my life activity damaged by this?

Thanks anticipatelly for your help!
Bob