My first reaction is, "Can you trim it down to 4500 words?"
At that length, you would have several options for selling to a genre magazine, at roughly 5-to-9 cents a word... which doesn't sound like a lot (maybe a few hundred dollars, depending on exact length and rate), but will usually be more valuable exposure than self-publication with no professional track record.
At the length you propose, you have a tough sell. It would be unlikely to be a viable stand-alone, unless you were able to sell it as adolescent/teen horror. You might be able to place it in an anthology, and there are a number of horror anthologies professionally published on an annual basis, so it might help to scout out the names of those working on such projects.
21,500 words is a lot more difficult to place than 11k, which is more difficult to place than 9,000, which is still more difficult to place than 7,000, etcetera. A few weekends ago, one of the best editors and anthologists in the field looked me in the eye and told me that if I could master 4,000-5,000 words, I would be able to sell everything I write. That was good advice, and I freely pass it along.