I TOTALLY feel your pain. I too have brittle & frizzy hair, which only 2 years ago was thick/full/bouncy. It seems to be on and off. I'll try taking certain vitamins, and then my hair looks amazing! Then a couple of days later it's back to frizz. So I figure it wasn't the vitamins. Or any other changes I try. It can be frustrating.
There's been alot of great responses here in this thread.....hope you try them and find a solution!