I have back pain myself and when I need to sit long hours I use a purple seat cushion for both my bottom and for my back.
If I'm in my office chair at home I will place a purple seat cushion on the seat and one behind my back and it makes sitting more comfortable for both my back and my butt.
You could also put a heating pad behind your back when sitting for long hours to help keep heat on your back which helps to alleviate back pain.
Also be sure you're sitting in the correct posture and position because sitting wrong could lead to your back hurting more and even your neck hurting.
Sit straight up with your back resting on the back of the chair and sit close enough to the computer if you're on a computer.
Also if possible get up every 15 to 30 minutes and take a break and walk around for 10 minutes or so because sitting too long is not only bad for your back it's also bad for your health.