I just caught this thread, and my heart goes out to you.
I know exactly what you are going through and how helpless you feel. We went through something similar almost three years ago with Rocky.
He was full of life and seemed perfectly fine. Then he seemed to have an upset stomach, and problems with his bowel movements. We observed him carefully for a couple of days, and he seemed to get over it.
We all breathed a sigh of relief, though at eight and one half years, and turning silver, we knew that he had lived most of his years.
Then, a few days later, the symptoms returned, and we brought him to the vet. Everything seemed normal, blood work, temp, and the vet couldn't detect any lumps after a thorough palpitation.
We got some meds to calm his stomach, and the vet was confident that it was a stomach bug of some sort. I left feeling much relieved, but two days later he was back at the vets, and that afternoon, I took a dog who could barely walk to the emergency hospital.
That was a Wednesday. The following Monday, we made the decision to say goodbye. Cancers had done their vile work, and we were not going to allow them to cause any further deterioration or pain.
We had said for years how difficult it was going to be when his time was up, but we had no idea how deeply we would be hurt. To this day, I get a lump in my throat when I think about him, and others going through this ordeal.
I will leave you with this thought. As difficult and painful this might be, it is because Tegan shas been such a special friend and family member. It hurts because you have shared a special bond with her. The depth of pain is but a weak reflection of the joy that she has brought to you and your home.
If it didn't hurt, it would mean that she didn't mean that much to you. So be thankful that you have had such a special friend. I feel worse for those who aren't deeply affected by the loss of a pet. They have missed out on so much.
Take care of yourself, and give Tegan a hug from Indy, Ava and me.
Tom