Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I am so sorry to hear that Missy is refusing to eat for you and is getting weaker.
At her age I would be very concerned that she has underlying metabolic failure (such as kidney or liver disease) that is causing her symptoms.
I understand at her age you may not want to do heroics, you are more interested in keeping her comfortable, but she may need some veterinary help such as fluids, and injectable medications, such as appetite stimulants and/or gastrointestinal protectants.
In the meantime please do not give aspirin if she isn't eating. Aspirin is a gastrointestinal irritant and can cause bleeding and ulcers even in a dog that is eating well, in a dog that isn't eating the effects can be severe and then she really won't want to eat.
If you cannot have her checked by her veterinarian today there are a few things that you can try.
At home you can give either:
1) Pepcid ac (famotidine) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and should help her stomach feel better and hopefully make her feel more like eating. You can crush these pills, mix with a little chicken or beef broth and squirt it into her mouth/down her throat with a medicine syringe. They are very safe and can be used long term if needed.
I also recommend feeding soft foods mixed with water or broths to make eating and swallowing more comfortable. If she is refusing to eat even soft foods you can try offering meat baby foods or a bland diet of 1/3 boiled, shredded white chicken or very lean hamburger and 2/3 boiled white rice. You can mix in low salt chicken or beef broth or warm water to make the food softer and easier to eat and swallow.
If she continues to not eat she should see her veterinarian for and examination, some blood tests, intravenous fluids and supportive care.
Best of luck with your girl, please let me know if you have any further questions.
I appreciate your advice. I will try the acid reducer with broth also. Thank You!