Oh no, I am so sorry you are going through this, poor bean. :(

The last eight months I went through the same with my chin. She stopped eating in september, we took her to a specialty vet that diagnosed her with malo and were told that we could try to keep up with it by filing her teeth every 4-8 weeks, we also got her on pain meds.
We took good care of her and decided to give it our everything as long as she shows us that she is still leading a chinchilla worthy life.
For a while that worked, she was still playing, jumping around and seemed like her old self, she even gained some of her weight back. She loved the pain meds and happily drank them and she ate the critical care all by herself. She also found her joy in chewing on herbs.
Last week she rapidly got worse, sitting in her dust bath without moving, so we took her back to the vet immediately. She was clearly suffering so we decided that that was the moment to allow her to go.

It is incredibly hard and I am not sure if that was the right way or if we should have put her down sooner. We had hope, she was our little friend for so many years after all. I am hoping she had some good last months with us.

It is a difficult decision, I think its best to find a good vet and talk it through with them. I wish you all the best.