I hear you bro. Although I have not yet experienced it with the car, I have had similar situations with my musical "career". A band breaks up and I can´t find a new project or people to start a new band with.
All my friends started new bands but don´t ask me to be in them and such things.
Then after a year or so, I got so sick of seeing my 20+ guitars on the wall and on their stands and my numerous amps and I started wondering if it was all worth it. Why not sell all the crap, make a huge chunk of cash and live happily ever after.
I guess it can be compared to what you are going through.
But eventually things get back on track again and you´re happy that you did not do it after all.
Now, you have certainly put a lot on your plate with all the mods you´re doing to your car but remember that you wanted to make it special. You did not want to do a half assed resto job so you could drive it on weekends. If you had gone that route, you´d always be wishing you had made your vision come true.
Now I have followed your work and your history for a while and I can´t start to imagine how you were able to pull it all off at the same time. I mean, come on, the computer renderings, the body work, the planning, the new job and most of all the taillights that you designed from scratch, had to figure out a way to make them, had to learn how to work the materials and last but not least make them.
Man that´s more than most people can pull off in half their lifetime.
So my best advice is to build your car like you eat an elephant: one bite at a time.
Don´t look at the things that still need to be done, look at what you already have done. Otherwise it´s too overwhelming.
Lemme give you an example: Each year, a guy dumps a truckload of firewood into my driveway and I will have to stack it in a corner of my property. Each year I look at that heap of wood and think that I will never be able to stack that.
But then I dig into it and I don´t even look at the heap in the driveway but I just fill my wheelbarrow and I concentrate on the new neat stack that I am building and how it grows and after a few hours I am always impressed of how small the initial heap suddenly has become.
That way you will be able to pull it off. Concentrate on the things you are working on, look back at what you have already achieved from time to time and forget the big picture of what you have not done yet and one day you will find that your car will almost be done.
Buying a new Mustang is definitely not the route to go. It´s a bunch of plastic that only happens to have a Pony logo on it. A new car, no soul, no value, no history. (Don´t get me wrong, I like them ok but they´re no comparison to a classic.)
Hang in there. You can make it. Work on that dream of yours. Giving up is not an option because the feeling of failure will never leave you if you do. Not after what you had planned to make this car into.