I think you would get more answers if you posted in Body & Soul.
I do sympathise, having suffered on and off for over 30 years. What I can tell you is that bouts usually recur, that when you are in the middle of a bout, it feels as if you always have been depressed and always will be, but with each bout you tend to become more resilient, and eventually, all though you still suffer, there is a sort of second part of you that acts as an observer and knows that it will not last for ever. Consequently, as you get older, the bouts tend to get shorter and less severe.