Hair repair?? HELLP!! Professionals only please!?

I have medium length, very curly damaged hair. I have been trying EVERYTHING to repair my hair and it is driving me insane! Nothing seems to work. I take biotin daily. I use Redkin smooth down shampoo and conditioner. 2 times a week i use Redkin extreme CAT in the shower.. leaving it in for 10 minutes and combing it through. I cant even straighten my hair anymore because it keeps breaking off. I use my straightener at a low temp. I use to use the superstar and it burnt out... then I bought a gvp and that made my hair ever worse!!! and a month ago i bought the CHI nano and that is also breaking my hair off! I have no idea what to do?! Even when i do straighten my hair I do it very seldom because of it keeps breaking off. Most of the time I let my hair air dry and I think that I am doing everything right to repair it. I don't understand what happened to my hair and why is keeps breaking off? I'm healthy... I exercise 5 times and week and eat very healthy so I really dont understand why this is happening to my hair and I need to know what else I can do to make it stop breaking off and stop being so dry. Thank you!