Irregular periods!

I'm in my early 20's and from the age of 15 I was on the depo shot! I came off it Nov 2014 so nearly 2 years ago, since I got my periods back they have been seriously messed up lasting anywhere from 16 days to 50! I've had LOADS of tests, even had an internal ultrasound and they can't find anything wrong with me. The doctor has told me that with how irregular they are that I may find it difficult to conceive however not impossible though they can't tell till I'm trying! Me and my fiancé will be starting to try for a baby soon and I'm really worried that I can't have children of my own at all! Has anyone got any success stories or advice, going through a similar thing?