This sounds Bipolar to me, Hoppery, whether or not you could actually be diagnosed as being Bipolar. I have not been, although one pdoc I saw suggested that I might be somewhat. I do know, however, that I have pretty-much always functioned similarly. I can cruise along doing well. Then something negative will happen & it just kicks the stilts out from under me, so to speak. Perhaps it is time to discuss this with the mental health professionals in your life.