The last time I had a problem like that, it was because my project had become overwhelming. I'd go into the garage and just stand there... not knowing what to do. So I took the advice of a few guys on here and did a couple things.
The first was to clean the garage. I organized all my parts and pieces. I put away all my tools. I cleared out most of what did not need to be in the garage to make more room. The idea was that the garage was now an easier, more inviting place to work.
The second was to list each individual project, then only concentrate on one project at a time. When that project gets finished, it gets crossed off the list, and a new project is started. That way, I stayed focused on only one project and didn't get overwhelmed. Plus I also got a sense of accomplishment by being able to see the overall progress by looking at all the individual projects that had been completed.
I actually bought a white board and some dry erase markers to do all this with. I still have that white board screwed to the wall in my garage, and use it still today.