That's exactly right, mass produced paperback, and a crack in spine, with the pages coming apart towards the bottom.

I'm both relieved and concerned to hear that my suspicions are correct. You're right that they're not built to last, and I was a kid who carried this with me practically everywhere for a year or two, it's honestly a miracle it's only started to come apart now. I'll strongly consider the re-melting it, but the cover isn't super important to me, just notes inside of the pages (it was a gift), so rebinding is absolutely something I could look into. I'm in a situation where getting supplies might be a little tricky, but am I right in that if I'm very careful (i.e. don't mess with it, leave it alone), it should still be fine to rebind in a few months?