edit because I can't spell, it's endometriosis and here's a link:
http://www.endometriosisassn.org/endo.html
Well you're trying to work it out, so that counts for alot. I've been in this situation before and yes it was VERY frustrating, but even more so because my ex was never interested in remedying it. Eventually I just gave up trying to get sex of any kind. It was a major factor in us finally breaking up.
Getting off the Depo should help in time, that stuff is about the worst form of hormonal birth control you can use, it has a list of side effects a mile long.
Have you been tested for endomitriosis? It is a very common problem in young women that often goes undiagnosed, they have to specifically test for it. Symptoms are the exact same as you described, painful sex, and sometimes pain when you're not doing it as well. Something like 1 in 4 or 5 women have it, but don't know it because they don't get tested for it, and its one of those things that only gets worse if left untreated. The pain intensifies and eventually it can impair your ability to get pregnant.
Good luck.