I need a hair product please?!?

I have extremely thick hair, so I blow dry it in layers with a heat diffuser, and then straighten it in layers. However, it dies so easily! After I get cut, I wash it after two days, I blow dry and straighten it, and it already dies! I do use heat Tresseme Resistant stuff, but it still doesn't work. But I can't leave it naturally down without straightening it. I was looking for a product, hopefully not to expensive. PLEASE HELP! thannks(: