Have you done a concrete analysis of how you did to get a rough idea of the range in which your December LSAT score might be? It sounds like you've thought about how many questions you may have missed, but a thorough analysis is in order for you right now. Here's an article that will help as you dissect your performance:
http://blog.powerscore.com/lsat/should-you-cancel-your-december-2011-lsat-score-2/Now, about your other question:
"Will it hurt me if I keep this score and it ends up being lower than my other score?" That's really up to each individual school. Although ALL law schools only report the highest score to LSAC and the ABA, not all of them look at only the highest score for admissions purposes. Some definitely do; Emory Law, for example, only looks at the highest, regardless of when it happened (as long as it's valid). Other schools look at your trend, others look at all the scores as well as the average, others look at all the score and when you took them. Whether a lower score will hurt you in the admissions process depends largely on how each school you're considering looks at scores, so it's hard to tell. A low score is never good, though, regardless of how you look at it--and a low score two years after a higher score is also not good, so there's really no rose-colored scenario or answer here.
Were I in your shoes and, after a thorough analysis to determine where my December LSAT score was, I realized that this most recent score was likely to fall 4+ points from my previous score, I would probably cancel. I wouldn't want schools to look at my December score and think that I had gone downhill since my previous performance, or that I wasn't quite up to snuff right now as I had been in 2009. Neither of which might be the case, but we have to remember that admissions officers are people, too, and they are likely to think things like this (particularly considering that you'll be competing against plenty of applicants that only took the LSAT once and did excellently, or students that retook the LSAT and got a higher score).
I'm sorry for the grim analysis, but I hope it was helpful. Perhaps some of the other regulars will drop by and give their two cents on your situation, so that you can have a couple more opinions to assist you in your decision! Best of luck with whatever you decide!