After reading your posting......it made me think....
I remember as a teen, we had to make the decision to end the life of our poodle, who had been in our family for 12 years......with sickness and injury, he got struck by a car. I remember going to vet and sitting with him......and as a family it was decided no to make him suffer anymore.....Of course, I cried my eyes out ......and walked away being selfish for wanting him to stay alive for me......
Whatever you decide, remember the Yorkie.....and then make a decision based on "quality of life"
I am sorry for your pain, I talk quality of life all day long in my job.......
what is hard, is when you really have to take action, and not just think about it.
Best wishes to you and your little Yorkie....I pray that you both will find comfort.