I feel I've tried everything but the gardenia my mom gave me is still dying. Any ideas?

Definetely a watering issue: since many leaves appear chlorotic I'd say overwatering and/or soggy soil. Basically the soil is too wet and it's both causing the root system to rot and inhibiting the absorption of nutrients and oxygen by the roots.

While there are all sorts of homemade cures for this issue personally I have never been able to save a single overwatered gardenia in my life.
The general cure you find in books and on websites is to replant in exceptionally well draining potting compound, trimming the roots if needed, don't water and leave the plant in a well ventilated, place in half shade for a few days to recover. As said I have never had any success with it but perhaps the gardenias I tried to save were too far gone.

I think that pot is too small. Watering issues can be caused by that, sure, but it would likely do better planted in the ground where it can expand and grow. (If your climate allows of course; gardenias don't grow very far north.) Give it lots more root room one way or another, and lots of good compost around... and if it can survive it will. Good luck!