My hair is cut in half when I tie it up. Is there an effective treatment to fill in the full volume?

My hair used to be so thick that I could tie it tightly with just two wraps of a hair tie, but lately, the thickness of my ponytail has shrunk by half; it feels loose even after three wraps. It feels like the density has loosened all over my hair, not just in specific areas. When I let my hair down, it looks lifeless and thin, and when I tie it up, patches of my scalp show through, making it impossible to take photos of my side or back. Hearing at the salon that my hair has thinned out so much has completely destroyed my self-esteem, and I just sigh every day while looking in the mirror. I think I might need to get a hair regeneration treatment. I was worried about my hair becoming thinner overall, so I would appreciate tips from those who received hair loss treatments at a dermatology clinic and saw their hair thickness and fullness restored.