[ Hair ] Open Question : How can I return my hair to its original state/thickness?

I used to have thick, long, black hair but unfortunately I went through a dyeing phase where I got bleached highlights and coloured my hair roughly 4 or 5 times in about 8 months. I'm quite sure it was all the chemicals in the dyeing stuff that has made my hair thin and frizzy, and now I want a solution of how I can get it back to the way it used to be. I also notice loads of hairs coming out whenever I brush my hair or wash it - I know thats natural but I never remember having as many fall out before I started dyeing my hair...! If anyone could help me, it would really be appreciated! Thanks, x