I think it is when you hit major down times in your life. Losses. I have experienced multiple times, but I know how to pull myself out of it too and that makes it into a wonderful experience. Or something good from the bad. Job loss at a time when you know it is difficult at best to get another. Divorce, a big feeling of failure. I started my education at that time and that made me feel like a success again. Loss of a parent, you tend to lose interest in your passions for a while. A little time and again a focus on something important and you feel better again.
Succeeding with my own business, that is probably the most difficult currently. What helps get past any negative feeling is analzing what isn't working and replace it with something that is. Constantly learning seems to help me out of just about any failure feeling. In fact, it has sparked some very new and great ideas that I may not have found otherwise; had I not failed.
Another one, letting go for a day. Nothing serious, just something fun, someplace exciting or working outside in my garden or flowers is enough to renew the strength and positive direction again.