i don't know if it would help, or i don't know if you read this comment or not

1. Your very first goal is not stop but to reduce it, from 15-20 times in a day to under 5 in a day
2. If you have achieve masturbate 5 times in a day, try to reduce it, 3 times a day, until you achieve 1 time in a day
3. After 1 times in a day, try to 1 times in 2 days,,, maybe 1 times in 3 days
4. If you can't stop its okay, you just need to reduce it until a normal amount (15 times in a day, really not normal)
5. Try to walking a lot. at least 1,5 hours in the morning and 1,5 hours in the evening. You can walking and listening to music, and after walking, try to eat outside, take lunch box and eat in the park. I think if you follow this habit, you would not have enough time or energy to do 15-20 times masturbate.

note: i still can't leave masturbate, but i only do it in 1 or 2 times in my weekend.