Have you been evaluated for bipolar? It could easily be something biological since you have had this problem for so long.

I agree that it could be from something traumatic that happened to you when you were 11 that you have blocked out. I have had this problem for years... blocking memories and being depressed for seemingly no reason.

I would suggest therapy as thats the only thing that has helped me sort it all out.Its good that you are in a loving realtionship because this can helpo you as you work this out.

Wishing you the best~