I’m 15 and I have all the same problems. My hair has been thick all my life, up until this year. I have lost a significant amount of hair, and it is noticeable when I look back at pictures. It has been falling out in large clumps when I shower, brush, or straighten it (which I don’t do very often due to damage). I went to the doctor and he says it is normal and that it will pass, but I feel like it won’t stop. I use castor oil on my roots. I feel like I take very good care of my hair because my mom is a hair dresser and she has taught me about hair care. So, I don’t know what is wrong.
If you are not happy with the doctor you visited, see another doctor. There is no reason to live like this.