I've dated a few girls that sound like your girlfriend...They want your time spent with them and not on your own. I had to let them go pretty quick.
You have to make yourself happy. If you enjoy working on cars, then that shouldn't change...Go do it. She has to enjoy her hobbies, too...If you both aren't doing that, then there's going to be friction, generally from resentment.
I gave up trying to keep a girlfriend happy by doing everything she wants. Now I go and do my own thing...If a girl wants to come along and be part of that, great. I'll go along and be part of her stuff, too...But if not, that's cool too - we both have our own things to do. As long as we're both cool with that and enjoy talking to each other about what we're doing, there's no problems. I'm fortunate that I've been living with a girl that works this way...Took a long time to find one.
Frankly, I'd probably say, "look, this is what I like to do...If you don't like it, either keep it to yourself or leave. I don't complain about the things you like to do, so try to extend the same courtesy to me."