I have/had this problem. My computer and shelves are full of partially completed (or, more realistically, just-begun) projects.

So, what I'm doing at the moment is I've picked one of them, and that's the thing I'm going to complete. When I'm feeling motivated, that's what I work on. When I'm not feeling motivated, I try to do at least half an hour or so before I flake off and go play games or work on something that feels more awesome at the time. At those times my motivation isn't that I feel that the project is worthwhile, it is that having gone through the process of actually finishing something will be have been worthwhile.

It's possible after I'm done I may never put that kind of effort in again, but I will know (a) that I probably can achieve that sort of goal if I want and (b) if carrying on to completion is hell, what kind of hell and what achievement would be worth it.