I am sorry to hear you are worried about Pippy. My name is Rebecca. I am a veterinarian and will do my best to help.
Does she vomit at all, or is diarrhea the only digestive problem?
It would help to know what blood tests were done and what the results were, if you have them.
Have you tried probiotics at all? I recommend ones made just for dogs like Proviable and Fortiflora.
I like the Royal Canin Gastrointestinal diet; it is the first one I try for dogs with digestive problems.
Since she is a Yorkshire terrier, I might check her for PLE, protein losing enteropathy. Was her albumin level normal on her blood tests? There is a fecal test for this condition.
Inflammatory bowel disease (IBD) is another possibility. To prove it, you do need to do biopsies with an endoscope or surgery. Sometimes I try treating for IBD with metronidazole and prednisone to see if there is a responses.
Since she is so painful, what pain medication is she getting?
Slippery elm can be used in dogs. It does not "line" her system, but it is anti inflammatory. If you try it, you would give 1/8th to 1/16th of the dose on the label for people of the particular product you try.
Pepcid is great for vomiting, but since it works by reducing stomach acid, it does not help much with vomiting. I would use metronidazole or probiotics for diarrhea.
Let me know what else I can help with or answer.