Okay, this might sound really silly... but I'm guessing that since sex/masturbation was taboo in your house, you never really got one of those "What's happening to my body?" type books meant to teach kids about puberty, their bodies, and sex. I had a couple in late elementary/middle school and I think they're great. I think you should get one or two books like that and read through them! You can go to a bookstore and pretend you're buying them for a niece or sister or something, or just order one online. It may seem juvenile, but these books are specifically made to present sexual health info in a friendly, non-threatening, no-nonsense way.
The reason I think this will help is that you need to sort of un-train your brain from connecting sex and your body with immorality. Your body is just a body, it is not sinful, and it is built to provide you pleasure. You need to learn to normalize that concept in order to remove the guilt from masturbation. Think of it like anything else you need to learn: adults who learn to read don't start with classic literature, they start with kid's books.
Edited to add links to some suggestions: I owned a companion book to this one and really liked it. This one is actually angled towards adult women so that would probably be a good read too.