I... I don't know if I'm doing the right thing selling commissions, am I making them too expensive perhaps? I thought a lineart at 5 USD would be good, but some people are already saying it's too much, I'm sad :(

That is definitely not expensive at all, especially for the work of your skill. The market is just saturated, and lots of artists are setting prices as low as possible since there's just a lot of artists out here. I do get the feeling tho... :(