Oh darn , the hardest part??? But all the parts are hard?
1. The writing. I actually really enjoy writing, but it's definitely the thing I spend the most time on, and is usually the thing I'm least satisfied with in the end. it's sad. Every part of writing— the pacing, the character's voices, keeping it interesting— I have to run all of it through tons of beta readers and cry over it and it still just is so hard to get right.
2. The flats. It's not that flats are hard to do— they're just incredibly soul-crushing and I hate them, so they take me forever to do and I'm miserable the entire time. I'm actually currently paying my little sister to flat all my pages for me because I just find it too painful to do myself.
3. Saying it's done. There comes a point in every page where I have to just say, "this is as finished as it's gonna be." When I cannot get my lineart right, I almost always just have to move on and color it anyways. I could keep rendering colors forever, but I have to call it quits at some point. Deciding where to do that, deciding when a page is done…. it's so freaking hard.