My hair is thinning and i’m not sure why!

The first two images are my hair about two years ago it was pretty thick and healthy although I heat styled it daily (I also had layers). The last photo is my hair more recently. It’s obviously much thinner (side note: I did not cut layers any time within the last year I think it looks that way because it’s very broken). However it doesn’t feel super heat damaged as I use significantly less heat than I used to. My diet is mostly the same and I would even go to say I’m much healthier now than I was two years ago. It just keeps getting thinner and thinner I can see a difference from even five months ago. I did cut it to shoulder length today. Any tips on how to grow it back thicker or shinier would be much appreciated!!!