Very Irregular Periods

TMI: I've had irregular periods since I first started getting them at 13 (I'm now 23) however they're getting further apart. I'm talking 2 months, maybe even longer. I used to get the symptoms (hormonal spots, sore breasts, minor cramps) but then no actual period would follow. And recently I've stopped getting the symptoms all together. I've never been able to track my periods because they're never on a schedule. Ive spent so much on false pregnancy tests because I just don't know why. I've been trying to adjust my diet and exercise as my doctor has said it could be a huge reason why. Ive been doing that for a while now and nothing seems to work and I think it might be more than weight but my doctor keeps telling me it's that and swiftly moves on. Does anyone have anything similar and have any suggestions of what I can try or talk to my doctor about. I'm stressing so much that it will seriously affect my ability to get pregnant later on in life.