Thank you for writing. I’m sure this is very worrisome. Since she is under a doctor’s care, the place to start is with whoever inserted the stomach tube. If that doctor doesn’t have an answer, please consider having her seen by a geriatrician. Geriatricians specialize in disorders found in senior citizens. It’s possible that your wife is dealing with the early onset of some form of dementia. It’s also possible that there is an underlying medical condition that is making her so depressed that she can’t do even the most basic kind of self-care.
Meanwhile, it’s also important that you have the support you need to manage the situation. Giving someone such total care is exhausting both mentally and physically. I hope you have family members and good friends who give you some time off now and then. Ask your doctor if there is a support group for caregivers in your community. Such support groups are invaluable. It is often helpful for people to talk to others who are going through similar things, both for the emotional support and for the information about resources and care that they are able to share with each other.
I wish you well.