You were right to write. When a fetish is on runaway like this, it interferes with a person’s ability to be true to their own moral code and to function in life. You are certainly at that point. You are not alone. Therapists regularly work with people like yourself.
Orgasm is a very powerful reinforcer. It’s readily available. It isn’t expensive. It feels terrific. Doing a behavior (in your case masturbating with pantyhose) can become more and more compelling because the person craves the “reinforcer” of the orgasm. It’s become like an addiction. Crack addicts tell me the same kinds of things. Although they know it’s bad for them, the anticipation of the high is so compelling (and the high feels so good), they can’t stop themselves.
You need a therapist, not a letter. You already know that you can’t handle this on your own. I encourage you to seek out a therapist for practical advice and for the support you are going to need to move away from what has become a destructive habit instead of just a way to add a little spice to your sex life.
I wish you well.