My hair used to be really thick but it’s not anymore

When I was a kid I used to have really long, thick hair. But it’s not thick anymore. And I feel kinda betrayed? Like people used to comment on my hair and tell me how nice and thick it was but no one’s said shit about my hair in ages lmao.
What changed that made my hair thinner. Am I not washing it enough? Or using the right shampoo? Or is my diet lacking in something? I really have no idea but I’m hoping someone on here does.