How To Treat Severe Hair Damage?

I have very damaged hair. Last year, i dyed my hair black to get my blonde highlights out. I dyed it brown that next weekend because the black due was coming out everytime I took a shower. I started straightening my hair every day. I stopped straightening my hair when I realize my hair was thinning out. That same year I started treating it and during the summer, my hair turned dark brown. It has grown in length a little but it has not gotten thicker. Everytime I cut my dead ends off they come back very quickly. Am I doing something wrong?