Having a hard time getting motivated is something that affects everyone, but creeps up on everyone at different times, rates and intensities. It depends on health, age, money, personal problems, etc. It's normal. But when it gets to a point where lack of motivation is a regular occurance, THAT is when you should start to worry about the rest of your life.
Whether it be working on your car for the street, show or race, it is actually good for you! The idea is to stay active, physically and mentally - as long as you don't push yourself past your physical, mental, financial and/or marital limits. It's healthy and the sense of satisfaction & accomplishment can be as rewarding as in the past. Be realistic on what you can do, how much you can accomplish and by when.
I'm 57 and plan to race for for as long as I can afford it, remain competitive and be able to do most of the work necessary on my cars. I have aches & pains, summer heat sucks and reaction times aren't quite as good/consistent as 10 years ago, but still pretty competitive. I do get "out of shape" in the off season and I have periods of a lack of motivation, especially when I have a string of frustrating losses. It can take time to get out of the "rut". But getting out of the rut and accomplishing your goals, like winning a race with what I built, continues to be enjoyable as ever. I've also had race car projects that I started with a lot of ambition, but found that I set unrealistic goals. So I started over with something more attainable. I hope to have a Dodge Aspen race car out late this season.
It may not be possible to do what you want until the very end. But until them, adjust things to help yourself. i.e: If you have 2 cars and realize that one will never get done, sell it and focus on the one you keep. Otherwise the second one could be weighing you down with worry and/or frustration. If you don't have the time & money to build a 9 second car, back up & build a 10 or 11 second car. If you can't finish a 100 point restoration, do the best you can, finish it and start enjoying it. Besides, there are a lot of upgrades you can do while still driving it!
Bottom line......don't give up too easily. You may regret it. Just be honest with your capabilities and plan accordingly. It can still be fun.