I know what you mean. I have to wage that war with myself, too, and some days it's really hard not to let the pessimism win (especially on days when I don't feel well). I just try to keep believing that I'm getting stronger by trying hard.
I really have trouble getting motivated. If I'm doing something for someone else, it's easy to feel inspired. But I don't have anyone to do anything for except myself so I just sorta don't care most of the time.