Kind of in line with what Mr. Chipmunk said, you gotta break it down to 1,000 small jobs instead of one big one. As a whole, its overwhelming...One at a time, you can do it.
I also take a lot of pictures as I go, when I start to feel discouraged I look back over how much I have got done, the difference from Day Zero, and it usually gives me the little push I need to get back at it with enthusiasm.
Also, I have found that I am wildly optimistic as far as estimating how long any given job will take....It always takes longer, usually MUCH longer, than I anticipate. Don't really have an answer for this, but just being aware of it helps.
Don't be afraid to try stuff you have never done before, almost everything form welding to painting to rebuilding a motor is not as difficult as it seems at first. Its very satidfying to have done it yourself, even if teh results are maybe not quite as good as you would have got farming it out to professionals...
That said, after this truck gets done, it will be a while till I take on a project this ambitious again! My next project will be running and driveable when I bring it home, like Cletis described...