Need to quit PMO ASAP!

So, I've been addicted to PMO since I was 10~11 yo. Today I'm 22. Half of my life I've been struggling to quit it, because I always knew PMO was something harmful, disrespectful and that it affects my self-esteem, confidence, motivation, etc etc.
I've had some successful streaks - 1 month, 2 months, even a year! - but I always come back to square one, I always relapse and it gets harder and harder to keep long streaks.
At my 18's, I discovered what real sex was, and instead of making me forget P and focusing on the real life, it made my urges even stronger, because of the memories I have, situations I remember and so on.
Nowadays I'm engaged, I really love my fiancée, but I also consider myself an addict to sexual behaviors, PMO and also sexting with other girls. It makes me hate myself... I feel like the worst person ever, because it is not that I want someone else, is that I feel that my body asking for more and more.
I just want to stop... I just want to be loyal, I just want to have natural sexual thoughts, relations and talks with my beloved one.
It is so hard... my mind triggers me.
I've read some stuff about the Coolidge effect, and it defines me. And PMO makes it even stronger, the need of having hot talks with different girls, watching hours of P, then MO'ing.
I'm trying, I go to the gym, I try to run away from bad thoughts, try to do things with my friends as much as I can, but somedays, even at work, at the gym, doing anything else, my mind is full of sexual thoughts. Even doing something important - in public, with other people - , I can't stop thinking about other girls, of P scenes, and so on.
I will never stop trying, but it makes me sad, that it is not the real ME who wants and fantasize with other girls, however, it makes me feel awful...