sounds dangerous! did she ask you already what your favorite horror movie was?
no seriously: if she doesn't understand - or doesn't want to understand - then simply tell her that you don't like to be around people that are always complaining and can only think about the past, that you are a positive thinking person and you're glad that the past is over or stuff like that. that you liked her the way she was (happy and positive) but not really the was she's now.
probably this is hard to say and she won't like it at all of course, but if she starts to scare you, you better get rid of her as soon as possible, even if you have to be a bit rude.
that's my opinion. but after all, you have to do it, so you have to decide what to do, not me. i think that telling the truth - even it's the hardest option for both - is the best way.