Vehicles, oh my god, I hate them. They're the worst things ever, I can never get their shape right, and if I manage to do that the proportions compared to it's surroundings is always wrong.
A problem most of the people here can't empathize with. Perspective where the horizon line is off the page, so I have to tape on another piece of paper is a pain, and if I'm not careful enough the paper will move which totally messes everything up.
Lastly, cityscapes. I hate drawing them, there's so many straight lines and little details. The people, the cars, the lights, the telephone poles, the trash, the businesses, the architectural designs. There's so much stuff I tend go into sensory overload trying to draw them accurately, but at the same time I want to get better at it and is the whole reason why I set my comic in a city. Man, when you get that stuff right, it can look amazing.
Not sure if you've done this before, or if this isn't to your interest, but you should do a Bob Ross marathon while drawing. It helps me to listen to his overly peppy, calm voice when I'm feeling particularly shitty, and unmotivated. Going for walks helps too.