I feel your pain and dejection! I just finished my first PT after completing the core curriculum and scored a flaccid 1 point higher than I did on my diagnostic, but 10 points lower than my target (I actually came to the forums to see if I was the only one with that kind of deflating experience). I'm obviously in no position to offer you any meaningful, LSAT test-specific advice at the moment, but I would tell you that, if your situation is analogous to mine, the urgency you're placing on yourself to apply this cycle is almost entirely self generated. I signed up for the Dec 2017 exam same as you; shelled out cash on applications same as you; told friends and family that's when I was taking it [probably] same as you; and convinced myself that I could be ready to score at/above my target LSAT score to get accepted where I want to go this application cycle. The shitty reality I'm now coming to grips with is that I couldn't and I also can no longer harbor any delusions that I'll somehow magically be able to by Dec 2nd. Conclusion, I will withdraw from the Dec. LSAT (it's the penultimate day to do so for refunds or test date changes) and withdraw my application from consideration at the universities I've applied to by email, figure this f*&#ing test out between now and this time next year, dominate it and go where I want to go. The alternative we have is to almost certainly underperform on the Dec. exam and prime ourselves to settle for a school we'd otherwise avoid or worse, to become dejected enough after a poor performance and the accompanying letters of rejection that we abandon the whole endeavor full stop. So what if I (or you) don't go class up in 2018 - really? Does it actually matter that much? Would I (or you) be more or less satisfied waiting for another year and knowing that we did what we set out to by achieving the requisite credentials to attend the school/s of our choosing instead of the schools that a hasty taking of the LSAT sorted us into? I know how much effort goes into this process and how eager I am to move past it, but I want to move past it on my, not the test's terms and if that means frustrating my timeline, prolonging the pain of preparation, or "disappointing" my friends and family then so be it. What's a year for a career? Be sure to ask the right questions of yourself before you commit to a decision and whatever way you come down; I wish you the best of luck fellow warrior!

It turns out that LSAC just extended the late registration deadline to Nov. 7th. Per the dashboard page on my LSAC account:

"The late registration deadline for the December 2017 LSAT administration has been extended through Tuesday, November 7, 11:59 PM (ET). The late registration fee will apply. Some test centers may be full, but you can add your name to the waiting list to automatically be assigned to a test center after November 7. For additional information, please call 215.968.1001."

So you can still crush it now (December) or you can crush it later; just make sure you crush it!