Cut back on watering indoors and keep it under lights for best results. This plant loves full sun in summer so it will benefit from as much light as you can give it in winter. Also remove all bad leaves to keep any infection or infestation away from healthy leaves. You may have to cut them all off if this is the issue. Spray with fungicide/pesticide if needed. If it is mites, you should see some fine webbing.

Take cuttings for insurance.

OR get rid of it and buy some new ones in spring.