I'll be honest here: I experience this feeling of guilt after masturbation as well. The odd part is, I never experience it after sex with someone else. I've never been able to understand it, except to think that maybe my masturbatory fantasies, which are just thoughts in my head, are probably unconventional for what I might think is acceptable is real life. The fantasy gets me off, but after I think, wow, I shouldn't have been excited by that. However, in my real life, what I do is "normal" and there is no feelings of guilt.
So at heart, most likely that feeling of guilt is related to believing that what you are masturbating over is wrong. You can manage this by either reminding yourself that fantasy is fantasy and as long as you don't try to go overboard with it, it's healthy to confine it to your masturbation, or you come to accept aspects of yourself and figure out how to appreciate it in a healthy way.
See now man....I'm kinda the opposite. I know I'm probably going to get laughed off the board... but the first and only time I had sex, I had such incredable guilt....well I haven't had sex again. I don't know if its something that church instilled in me or what, but I was sick with guilt for a week or more after my first and only time. Idk, but all I know is the next time I have sex, its going to be in the confines of a deep, long relationship with someone that I care for and love. I know I'm weird