my brain won't shut down
That is the root cause of the problem, and we have to consciously win over that, telling the brain not to think anything after a time. So a little discipline to start telling your brain that it's time to stop, before actually going to sleep. However, even if you do that sometimes the brain does not listen to your heart. And that is where exercise will come in help. Since you are not too old or fat, you must get back into exercise - a 20 minutes walk would suffice to start. Exercise has tremendous impact on our health, its difficult to do it dedicated.
I have gone through all this, and still dealing with it. My root cause was having another reason - drinking too much water...Know your body, and I think, you will sleep well. And tell your brain that, it won't survive without the body, so don't be stupid to not allow the body to take rest.