Depo is pretty horrible and it's surprising that the FDA hasn't banned it by now. It does have lasting side effects that could cause problems for more than a year after stopping it. I'm just guessing here but I think it's possible that since you're still developing and haven't gone through puberty when you got the shot, it's having more of an effect on your reproductive system than it would for someone older.
You said that you've been to a doctor but what kind of doctor was it? If you went to a general practice doctor then you're not going to get a good answer because they don't know anything about reproductive health. If you haven't already gone to an actual OBGYN then it's time for you to go see one. They are specialized in evaluating these types of problems and would be far better suited to helping you figure this out.