Bit about me first to see if you can compare your situation to mine. 21yo, bad masturbation techniques (i.e trying to come as fast as possible to avoid detection). Late sex starter (19), majority of sexual encounters were drunk. When sober, I'd cum in under 1min.
I beat it by getting my 'mind right' while using stop-start each time a masturbated. The material in my brain when I masturbated wasn't 'real enough' so I was getting used to getting off to a tamer view. I had to become more comfortable with the real deal, and to be honest each time I thought these 'real' thoughts I'd almost cum on the spot. I felt a little uncomfortable with these graphic thoughts. That was a problem. So at free moments I'd think of the most graphic sex !**@! I could while breathing deep and slow to keep myself calm (just thinking, no touching). It may sound obvious, but give it a go and if you feel yourself coming up against a bit of a mental wall when you try to think up some dirt, you might have the problem I had.
Pro tip . Get your imagination dirtier and become comfortable with graphic sex in your head. Sounds easy, but you're tyring to change your whole way of seeing sex. Can take a few months. Also, when masturbating, always use some delaying technique like stop-start or masters-johnson.