It was a tough decision, but I decided I'm going to revise my novel. After doing some soul searching and rereading, it's really not that bad of a story, it just needs a few tweaks. I decided I don't really care what the agent says about zombies falling out of fad, they always say, "Write what you want to write." I've read numerous agent/publisher blogs that say you should never write for trends (which is extremely contradictory to what they actually practice because they usually only sign authors they know they can sell, which has to do with trends; it's a vicious cycle), so I'm not going to. So what if the zombie trend is slowing down? That's what I like to write. If you can't sell it, fine, I'll put it away and work on something else. Maybe one day it'll come back into vogue.
I'm too stubborn to give up on the book. And I'm actually enjoying going back and working on it. I thought it was going to be tedious and I was going to dread it, but the only thing that's freaking me out is sending out queries. I've decided I'm going to put it out of my mind. The revised version isn't done yet, so I don't need to worry about that step. Perhaps in a month or so when I'm done, I'll be able to approach the subject with a different attitude. If not, I'll move on.
Since I've decided to rewrite the novel, I pulled my submission from the indie publisher (who has had it for over 270 days). Perhaps later down the road I will resubmit to them (perhaps). I was still under consideration, but I wasn't happy with the story. This way, I don't have to worry about them or wait for a reply. I'm free to do whatever I want!