I can be a bit streight talking so please don't be offended.
Did you know orgasms are good for you? In a similar way to crying is too. Having an orgasm can release endorphins, reduce pain and relax your body entirely. If that doesn't sound like a benefit then think of the calories burnt. Sex and masurbation can be part of your health eating and more excersize regime. An orgasm a day keeps the Dr away. . . . Ok so maybe not, but it's got to more fun than an apple.
You have no reason to feel dirty do bad about masturbation. You mention you have a bloke. Could you masterbate him, (hand job), you could learn what he likes, then maybe he could do the same to you. Using toys if you prefer, this way you can associate the feelings and pleasure with being with a man, as I think this might help your mental parachute to masturbation.
As others have said being relaxed is crucial, if your not feeling horny or sexy don't bother, it's probably not going to work and might make the problem worse. Im a woman for a practical solution, so have you tried a set of jiggle balls? They can be very inexpensive, but wearing a set might stimulate you a little. So your not going to orgasm from wearing them, but the free roaming balls inside might remind your body that they are there, making you feel a bit horney or sexy, maybe even wet. I get very turned on with mine and almost every time I have worn them (which is almost daily) I will masturbate or have sex with my OH. It might be worth a try. But check the sizing, if you have not had children not too big will be better.
I hope this helps.