Hair is falling out - please help!

I recently turned 40 and I’m noticing a considerable amount of hair loss and hair thinning, and it’s starting to stress me out. I’ve always had thin hair but it used to grow back quickly. Now I’m definitely noticing the loss and I’d like to stop it before it gets worse.

Has anyone actually had success with anything? What should I do? I do not want to waste money on stuff that doesn’t work. And I do not have any underlying health issues that I know of that could be causing it. I am generally in good health.