So whenever I plug in my charger, the light on the side turns red and the charging battery sign pops up, as normal, but as soon as the calculator turns off (which it does automatically after inactivity), the light turns green and it stops charging. I have left it overnight and it's still saying low battery. I have also tried to keep it on by using it a lot so that the red light stays on but that's not working and it goes back to low battery when I unplug it. Resetting isn't working either. I've had this calculator for about 1.5 years and it hasn't had any problems with charging until now.
Please help me, I have an exam in 3 days graphing calculators are expensive so I don't want to buy a new one unless I have no other options.
I don't think the charger itself is the problem, but I could be wrong :/

Do you have an extra battery? If not, I'd suggest ordering one right now just in case we can't fix your battery issue. You can buy one from Amazon for about $12 Here. (The CSE and the Nspire use the same battery)

I looked around TI's website and found this page about the CSEs battery. They provided the number 1-800-TI-CARES for help resolving charging problems.

Fortunately, you won't have to get a new calculator, but your battery may have reached it's life expectancy. It's rated to last 3 years, but it's possible that it was slightly defective and died earlier than expected. I'd suggest talking to TI about that issue though.

You can find the guide to replacing the battery Here. I know it's about the Nspire, but it's the same concept.

Good luck on your exam!