Set a timer, do it for just 20 minutes, and stop, stop as soon as it goes off.
Then, no closer than two hours from then, do it again.
Try that a few times, until you’ve managed three 20 minute edges in a row, and I hope you’ll start to feel the ‘denial high’ the benefits of not cumming, that you feel good, ALL the time, not just when masturbating.
Because really, learning that, discovering that as good as an orgasm feels, it’s just a few seconds of pleasure, compared to feeling good all the time, is what really helps you stop.
Cumming is good, not cumming is even better.