Feel it so much for you. Hard decision to make. Most of us on here will have had to make that hard choice. All I can say is you have given her the best of times and now you are doing what is best for HER.
I have experienced it with several animals. Never gets any easier. In fact my amazing mum died in January and I cried buckets and then my beautiful cat became ill in April and I had to make the decision to say goodbye to him. I cried even more (if that is possible!) I wrote on here and got a lot of support.
It is never easy but prolonging her life would only hurt her more.
Just be prepared to grieve as you do for anyone you have loved unconditionally for so long.
My thoughts are with you tonight xxxxx
NHS U turn on trans terminology