My hair has radically changed...Help!

I am a 45 year old woman who for most of my life, until recently, had thick, naturally curly hair which I have loved! Hairdressers marveled, men swooned, etc., etc. Now, in the last 2 years or so, my hair has thinned so much, it’s TOO thin. Also, in the front, it’s to the point it’s almost so thin you could call it on the edge of balding. My hair is more brittle, more dry, and the curl is all but gone. I don’t use hair dryers, didn’t need and don’t use permanents. I’m kind to my hair. But I really HATE my hair now. Nothing looks good. Different cuts and styles,to make it look fuller, fall flat. It’s lifeless. Help!

P.S.: I am not on birth control any longer, but once was. I have had my thyroid checked. No problems there the doctor claims.