This is gonna sound a little weird but how do you stay motivated/improve? I've been doing digital art for a few years now but I always get so frustrated doing line art and tend to just.. give up after a while, and your lineart is so nice and smooth.
I actually have a ton of trouble staying motivated too. Sometimes I have to just force myself to get through a piece. In my experiences so far though I can easily say drawing what you love helps a lot. It’s weird but its totally possible to take a break from art… with art.
Tired of drawing one thing? Just draw something you’ve been interested in recently, experiment with some techniques you’ve seen around, if you do a lot of personal art do some fanart, and vice versa. If commissions or previous projects lose their spark for a bit, you don’t have to finish them IMMEDIATELY, take a break. As for my lineart I do really well with quick short lines, personally. It’s kind of a mix of a clean final sketch in terms of production? If you look closely you’ll see the lineart is a bit messy, but I use brushes that easily hide mistakes. You can see them here!