Don't worry about length, really. 90K is also a very genre specific number. I don't run into writers outside of the genre worrying about these numbers, and I think it's a shame that writers in the genre feel compelled to reach for a number of words instead of just writing whatever length a piece should be. If it's an 80K novel you write, you can sell that too. There are lots of them out there. There are even lots of 70K novels out there. And not just YA novels, either. If it's good, people will hear about it and buy it.
If you're 18K into it, you've got a good start. Keep going. Don't look back. Just run with it like you would a short story. The only difference is you have to figure out how to pace yourself for a novel run. Short stories in an hour rush. Novels can be written in a rush too, but it's more like at least a few months (for very fast novel writers--obviously not me) to a year or two.
What works for me is this: you've probably heard of the page a day method, right? 365 days a year, one page a day and you've theoretically written a novel. Well I don't like to write everyday, so this doesn't work for me, but I do like to get about 7 to 10 pages a week written, which is the same or more than what you'd get if you did a page a day. So I usually write every other day at this point, and write anywhere from 3 to 5 pages in a sitting. It feels doable to me, quick but not rushed, and I get the same amount of pages done (or more) each year. So if you don't already have a pace established that you like, consider something like that.
I'm excited to read this. But from my experience, you'll hit another hard spot as you near the end and have to start drawing what you've set into motion to a close.
But yay! Book!