My Cat Has Several Issues

I have a 15 year old Cornish Rex, who is very picky.  She has had upper respiratory problems, and was very sick recently.  I have been to the Vet many times.  He said she has allergies, and to try Lysine.  I tried the Lysine in the tube, and I have torn the Lysine chewies into pieces, and put them into her food, but she knows when her food is different, and won't eat it.  I have ordered powdered Lysine, but I am skeptical.  For the past week or so, she has been sneezing a lot, and it sounds like she is trying to get something out of her nose.  She has also been crying all the time, and the only thing I can do is pet her.  She has been making some loud crying sounds, that have scared me.  Now, she won't even eat her favorite food, with nothing in it.  I have tried everything;  different foods, cooked chicken, but she won't eat.  I'm worried.  I don't think taking her to the Vet again is going to help.  Does anyone have any suggestions on what to do?

I agree, taking her to the sme vet might not make a difference. Id take her to another vet though, for a proper checkup and a second opinion.
What are her alergy symptoms? There are more effective things than Lysine to try, to see if she responds, and therefor makes allergy a likely diagnosis. Allergy shouldnt be the sole reason she isnt eating. Considering her recent URI id have her checked if she either still needs help in fighting that, or if it could have spread to her lungs and bronchies. That she would need help fighting off. Cats with stuffy noses tend to eat badly as they often get triggered by smell to eat. She might not even realise its her favourite stuff in that food!  If she has an infection, she might also feel generally low, and it affects the apetite. Get them to check fever,a full hemotology, with bloodcell count and the lot, even x-rays of her lungs. Dont let them write it off as an allergy just yet.

Agreed, would definitely have them check for a URI. My Luck had a really bad cold earlier in the year, and a ten-day course of antibiotics later, he was SO much better. He was sneezing like he had something stuck up in his nose and also had this horrible cough. He surprisingly didn't seem to lose his sense of smell despite having a stuffy nose, as his eating was never impacted by it.