I believe it could be about the high temperature + daily exposure to sunlight. I used to have those kind of hair problems where I occasionally digital perm/straighten my hair. The worst occasion was when the saloon misunderstood my intention of hairstyle and had my whole hair 'frizzed'. You know the kind of style that goes 'zig zag zig zag'. Yeah that's bad.
What I did was that I waited for the new bunch of hair to grow and have the rest of the ugly ones cut off. Yeah the wait was excruciating but I couldn't do anything to help it. Not even shampoos help. I guessed the 'broken' once are just 'dead' and you'll have to wait for fresh ones to grow.

Just from experience, even though I do still blow-dry and straighten my hair, I make sure that it is at the right temperature. Better do it right then to have unhealthy hair later on.

However, I do have one problem with my hair that just happened recently. My hair ended up looking 'oily' after having no choice but to use a dandruff-free shampoo. Tried shampooing it 'off' with my regular shampoo but still it persists.

I was about to start a new topic but now that I find it relevant to yours, I figure that I could ask for advise here. I am sorry if I trespassed.