My back hurts so much from sitting down for long hours. What should I do?

Be proactive by. Checking to make sure your seating has armrests and a good back support and possibly adjustable in sitting height. Even then, make sure you get up periodically to breakup the time sitting. Consider exercising -emphasize core stengthening and keep trim along with avoiding smoking.
Lumbar support. I would get more lumbar support and make sure the chair is the right height so your posture is not compromised.