Terrific subject. And relevant to my interests since my turbo e30 build just fired up for the first time a few weeks ago after being off the road for 2 years. I honestly felt anxious about starting it because I got used to it not being done, I was almost not sure what to do with myself if it actually fired up.
I always run into a phase in a project, sometimes more than one, where there is often relatively little left to do but it ends up taking forever. Part of it is because none of it interesting - such as after finishing all of the mechanical work on an engine swap, only having wiring and some plumbing to do. Another part of it is that I'm maniacally task oriented and usually none of these tedious things can reasonably be finished the same day after I start them. So I know for a fact that I won't be "accomplishing" anything today. It's false but there's no convincing my brain.
The only cure I've found is to just go. I don't want to hear about having no time. Everyone has a half hour they can dig up. Get up early and do it before work, do it right when you get home, or just before bed, it's not impossible. One of my other hobbies is powerlifting. After a full day of work, having to look forward to mustering the focus to put 700 lbs on my back when all I want to do is eat, watch TV and go to sleep, is daunting. I know that the second I try to come up with an excuse to delay going, I will succeed and it will usually end up into a perpetual delay. The only way it really ever works is to get home, change, grab the gym bag and go.
I find it's the same with the car. If I want to come up with an excuse, it's easy. But if I stop thinking and simply start acting (change, walk into the garage, turn the music on, pick up tools...), things just happen.
Another thing to consider is that it's perfectly natural for interests to wane without actually disappearing. The most important thing at this time is to realize the latter. Just because you've lost interest in a hobby, doesn't indicate that it's time to sell everything off and get out of it because you will regret it, horribly. One day, the interest will return like getting hit by a train and you will be left thinking "what the hell did I do...".
So if it's a boring period in a project, grind it out. A bit of time, regularly, without thinking - just go and do stuff. If you're losing interest, put it away and DON'T TOUCH IT. You'll want to get back to it in time.