Too active a mind is the most possible answer. Learn to relax & let go. Don't put the weight of the World & its complexities in figuring out solutions upon your own lone shoulder...
You may have to trick your mind. Instead of focusing your mind to sleep, tell yourself to just have a rest. There's a psychological difference between the two. To sleep when you mind does not allow you to do so, due to high mental activity, you would fail.
Instead, should you tell yourself to just REST, not sleep, the mind instead will focus the Human body on relaxing muscles from toe to top, even the mind, & eventually, you will drift off to sleep in such a mental state.
Just let images, real or imagined, flow thru your mind, with no disruptions or attempts to resolve issues which seemed real but only a mental conscious reminder & reflective of the tornado in your mind.
Give in to the nonsensical images instead, Such would need no mental ENERGY to process, & you will sleep within minutes.
There! Had I put you to sleep with my long winded & rambling post?