Is My Cat Gonna Be Okay?

I ve had Tinkerbell, my cat, for about 13 years. She s a white cat, but doesn t appear to be going blind, at least, from what we can tell. Tinker use to be super overweight, but with old age, she begun loosing it (also do to a bigger house and running around more than being lazy). It was great, she was finally at a healthy weight. Cats are suppose to weight around 8-10 lbs. In the last couple months, her weight had been dropping fast. So we took her in.. She weighs 4 lbs and even her bones are noticeable. They found a, if I remember right, thyroid tumor in her throat. They have us medicine. We gave them to her for about a week, but it was killing her, she wouldn t eat anything and she wouldn t respond to anything or anyone. So my parents made the decision to take her off of the medicine. She s slowly beginning to eat, but barley anything. All she does is sleep, she s sleeping on me at the moment, and it s the first time she has come to me in months. I noticed today, she has a huge bald spot on her throat (if the picture is allowed I ll show it) and she looks miserable. I don t wanna have to put her down, it would kill me, but if she s suffering then maybe I need to? Any advice? She s also constantly fidgety, her ears, her mouth, always twitching. How can I help her??