And this is what happens when I watch some videos about proper mixing and mastering in FL Studio:
I didn't re-record anything, just removed all of the crap I attached to each track and gave each instrument its own EQ (most of them just got simple low pass filters, a couple were more intricate), then mastered the whole thing. I could probably get it even better if I let it sit for a few weeks and went back to it.
The worst part was hearing each guitar track in isolation -- my playing is sloppy, but I didn't notice three years ago because the mix is so muddy. If this wasn't ancient history, I'd be inclined to scrap everything and start over, but the point of this was just to practice mixing and mastering. I'll be remastering the entire album in the coming weeks. When it's done, I'm replacing the files on my Bandcamp page. The idea is that I want to make these skills second-nature so I can focus on new material (I've started a few things, but I keep letting myself get distracted by trying to EQ things before they are done, which is like editing your sentences while you're writing. Bad Stan!)