I think at my core, what I do is borrow techniques from others or discover completely new ones on my own when just dicking around. I don't really do exercises, though I bet if I did I'd probably get bored with scales and mess around with them. Maybe having ADD helps. Your own style is something that develops or changes over time. I don't think I've ever actually TRIED being derivative, a lot of my music is still just accidents that happen while experimenting. I don't really go into my sequencer expecting many specific things to pan out.
You said you're big into jazz so I dunno why you can't just pull something out of your ass like I do . Being a perfectionist might be hurting you more than helping, especially when it comes to realism of sampled instruments. Keeping down this path, you'd really only be halfway satisfied after spending lots and lots of money.
The best thing I can tell you to do right now is to join a compo, on IRC or in the forums [nobody reads the "competitions" section, I guess] because there are lot of 1 week or 1 month [or one hour!] compos that will force you to develop speed-writing skills, and how to "critically think" about your music in a short amount of time. That'll force you to abandon any perfectionist qualities and soon you'll just be able to write. Enough compos and you'll make MIDI sound like this without even thinking about it. Of course not many compos are MIDI anymore but lol nario!!!!!!111