I bought a seat cushion. I can't log into Amazon right now to show you the exact one, but I think I paid about $30 for it. Mine looks similar to this:
https://www.amazon.com/Seat-Cushion-...7HD8B9SP&psc=1 I couldn't believe how much that helped with my neck, back and shoulder pain.
Regarding headaches, it sounds like eye strain. You may need computer glasses.
I hope you find some relief soon!