Premature ejaculation has nothing to do with masturbation. If anything, frequent masturbation should cause the opposite effect. Although I think it's all in your head, try the stop and go method. Look it up. If you do it right, it should take a few weeks and you'll start to feel in control again.
I agree w/ Jake - in fact, you can use masturbation as a way to learn to hold off on orgasm/ejaculation longer. As Jake says, use the start & stop technique. Start stimulating yourself, but as you get close to orgasm, stop & let yourself cool down for a min or so, then start again. Each time you get close, stop & wait a bit before resuming. You can do this several times, and then go ahead & allow yourself to ejaculate. Over time, you’ll find you can ‘train’ yourself to last longer. As a ‘plus’, your orgasm will likely be stronger & you’ll prbly ejaculate a larger volume...