I hope I'm not invalidating your experiences or the way you view how you feel. I've had bouts of depression since I was four years old although they weren't diagnosed until much later. I have felt many times that it was over... it not always having the same meaning as the previous time. I lost large periods of my life due to MI. I would think... it's over. But then things would turn around and I'd accomplish things that had seemed to be the "it" or part of it, that I had thought was over. I guess I should say the accomplishments weren't what I'd envisioned as my life dreams but they were good... sometimes close to what I'd expected to do in my life.
I suppose my thoughts now are that while we may not accomplish what we wanted to do, you can never know that it, meaning a productive life, is necessarily over just because of present circumstances. It is true that a lot of doors we wanted to go through become closed to us as we get older but there are many doors in life. I hope that helps. I hope it makes sense