Hmm. Well. I'd look at it like this. First I'd have broken up with her the first time she cheated on me. Next I'd sever all ties with her and give her the cold shoulder. I wouldn't deal with that crap, dude. And as for your current situation, focus on your current girl, and ignore the crazy who's bugging you. Let your current girlfriend know that despite your ex going nuts on you (Maybe she has some strange obsession and wants you back?) that you're not going to let it ruin your relationship with her.
Or go up to the crazy girl and work that pimp hand! KER SLAP! Just joking, of course.
Good luck, man!