There's only so much 'we', as in this website, can do for people. Half of the 'cure' for this 'sickness' is one's own will to find the answers. You have my deepest regrets for the turn your daughter took, but perhaps this is for the best, if you look at it in a more positive manner.
When 2012 comes and goes, the next big doomsday date will arrive. She may fall prey to it with out professional help, if you understand what I'm getting at. Still, there is no reason you cannot help her now, still. Show her this website. Show her how ludicrous the entire 2012 thing is.
The excuse of 'a typical teen' is not enough to warrant deep, dark depression. Clearly this turn is something she will need to gain the strength to see through falsehood and lies. A lot of us here did become emotionally disrupted, if not destroyed, over 2012. We all, however, had a common goal: to seek the truth. It is up to your daughter to break free of her darkness, to discover the truth, and to grasp it.
Hold her hand for now, and help her through this. But, as I've said before on my stance, a victim has to eventually walk on their own to be able to avoid this sort of thing happening again.
And as for you? Don't worry. You'll be having fun with the nightmares of her turning 21, drinking, and driving soon enough!
PS: That was an attempt at humor. Cause, you know, she will be turning 21. Yeah. 2012 is stupid. OH HEY LOOK 21 IS LIKE 2012. WE'RE ALL GONNA DIEEEEEEEEEEEEEEEEEEEEEEEEE!
Support "Moo's Conclusion That 2012 Is Real!"
PSS: You'd be surprised how serious some people will take the above PS and support it as a claim.
PSSS: This is the sort of stuff you need to show your daughter for her to break out and realize just how stupid this entire thing is.
PSSSS: If worse comes to worse, and if you got the time and money, go have her visit the Mayans. All 6,000,000 of them still alive and laughing/getting annoyed at the 2012 crap.
PSSSSS: These are getting annoying.