It may NOT be the end of her life. You don't know yet. All I can say is she is feeling pain. (thus the shaking/trembling )Her bowel movements are abnormal. She is not eating. She needs to see a vet promptly.
You need to find out very quickly exactly what's wrong with her, then get her treated if at all possible. See what the vet says.
The lump under her mouth may be anything, and it may not be cancer (?) It could be a cyst, or it could be a tooth abscess. That in itself would be enough to cause her extreme pain and make her not inclined to eat. If she's not eating, her bowels will not be working properly.
However, it certainly does sound as if she has some problem going on in her bowels too. A stool-sample test may help. And if you think she is up to it, an x-ray perhaps might determine the cause.
My condolences, sweetie, but I think it's time to put her down. Better to not let her suffer because she can't possibly be comfortable with what you're describing.
Oh dear. You have done such a wonderful job with your JRT to have him for 16 years. Unfortunately, the symptoms you describe sound dire. I'm not a vet, but I have seen this sort of thing in my own dogs over the years and I suspect your JRT is at the end of his life. Please take him to your vet. If nothing else, perhaps he can be made comfortable during his last days. :-(