I had a similar thing happen to me - I had access to unlimited work space, a good amount of free time, and progress came quickly. Shortly after I moved into a serious of apartments and could only work on the car after a 2 hour drive up north... progress crawled along and I really lost motivation to go up there. Now that I've been in my currently location with attached garage for a year it has still taken me a while to get back to it, and I'm not as far along as you having a driving car!
Some of these suggestions I'm echoing what's already been said, but for me what helps is:
1. Having a clean/organised/fun workspace has made a huge difference to me. Even if I'm not motivated to work on the car, the garage is now a nice place to hang out so it's easy to convince myself to head out there. Once I'm out there I usually find myself getting something small done.
2. Committing a little time each day to work on the project, or even just being near the project to make shopping lists, to-do lists, clean up, prep materials, etc.
3. Having a to-do list that's broken down into very small items. This can work both ways, if there are too many things on the to-do list then it can look overwhelming, but it's also extremely satisfying being able to complete something minor and cross it off the list. Don't rub it out! If the list is too long then keep it broken into small chunks but only write up the first half of the list, then when you're close to done you can re-do it. Whatever works for you!
4. Start a build log, if you haven't already, or even a thread on Garage Journal or similar. I really enjoy keeping a log of progress on the project and hearing feedback from other like-minds.
5. Tell friends about the project so they can hold you a little accountable - my friends always ask how the car is going and I find that helps keep me interested as well.
6. If you need a break from the big project, take it! This is a hobby after all, I like to have a couple of small projects I can swing onto if I don't have motivation to do more stuff on the car.
I sometimes have a real problem with motivation to work on the car, and other times I want to be spending every waking minute out there, the above tips are what has helped me.