How do I erase my masturbation-discovery nightmare?

When I was younger I was discovered masturbating by some friends who laughed at me and ridiculed me afterwards. It’s almost twenty years later but I still have shameful nightmares where I remember this scene and wake up crying and with a painful feeling that I was stupid not to protect my privacy better and that it was shameful to be seen doing what I was doing. In my dreams I sometimes get revenge by changing what actually happened so that after they find me I beat them up. I also feel guilty that I did not confront them physically at the time. I don’t know how to be free of this memory. Probably I never will.