Patient: Doctor, it hurts when I move my arm like this.
Doctor: Then stop moving your arm like that.
What makes me keep going on the painful things, the tasks that no longer provide the same reward, or the prize endlessly deferred? Why don't I just stop doing it, and work on what brings me joy? Some of my drivers are not so noble like the icy fingers of guilt, or the evil whisper, "Quitter".
On the other hand, there are reasons to carry on past the odds. Perhaps my gut tells me I am working on something I am very, very good at, and the reward will come even if it is not obvious right now. Even if few people confirm my instincts just yet. My passions tell me I have something valuable to contribute. This must be what drives artists and writers to work in obscurity. That, or indescribably large egos.
Another reason to keep going was taught to me by a lawyer friend. She lived for the energy of the courtroom, and she was a natural showman. She loved her job and she was very, very good at it. She pointed out that a career can be an exchange. She lived for her day in court, but for a lawyer, that is a brief culmination after days and nights of slogging research. She did not enjoy the slogging part. But she did it, and wore out a lot of secretaries (she deliberately hired people with strengths were she was weak), in order to do what she loved.
The trick must be to be sure about your passions and remember why you signed up in the first place. If the dream still holds, follow your intention (borrowing from the Pike's Place Fish boys) and the rest will follow.
Ouch, this hurts.