There is actually a psyche reason for your lack of enthusiasm and it is not uncommon so first off, don’t feel like you’re alone and second stop beating yourself up about it. Some people have mentioned the heat, humidity, freezing air, lack of funds, wife/girlfriend bitchin and so on. All of these add to the pressure that you already put on yourself. And even though you can see a lot of things that you have accomplished, the pile is still just sitting there and your brain actually starts it’s own questions about the project that you are not even conscious about.
A couple of things that can help you get back to it. Our reasons for doing a project can be numerous – we want to enjoy our hobby, we want to build a car and flip it to start another better project, we want to hang with our buddies and their rides, we want to see the performance improvements, we want to take our wife/girlfriend for an ice cream and prove her wrong. You need to sit with a beverage and spend a few minutes here – what was your reason for the project and has it changed? That process can sometimes rekindle the enthusiasm aspect but rarely will it be enough to keep things going but you need to know your reason. Another thing that helps is if you have any like-minded friends that might come over and give a hand with it. Sometimes having someone else around, even if they’re not mechanically inclined lets you bounce ideas or give you the chance to explain to them why you’re doing some particular item on the car that you’re proud of. You can also make yourself a project sheet, again with the beverage sit down and make a list of all the things that you planned on doing with the project, include the items that are done or partially done. You can break up the list in sections if you wish – engine, body, interior, wiring, etc. and you can do this in a notebook or a pad of paper. Then after you have it all written down, go through and check off the things that are completed – this signals the brain again on accomplishments. Work on your project in small sections and stay with that section until it is completed or as completed as you can take it to at that point. And one last item, move the project if you can manage it. Physically change the position of the project in your work space. This one simple move will give you a completely new perspective on your project.
The last two items are the ones that I use the most when I get a bit of burn-out. My current project is moving slowly but I have put together some assemblies and I have moved the car twice. Good luck with yours and I hope some of this has helped you out.