I am so sorry you are going through this. I can completely relate to how you are feeling. We had to say goodbye to our sweet Boxer Coco in August of last year she was almost 13. My husband and I did everything we could to keep her alive. We decided that when she stopped wagging her tell or responding to our touch it was time to let her go. One Saturday morning we woke up to her licking up her own urine
When my husband ran downstairs (she used to sleep with us but was more comfortable and cool in the basement) he said Coco and she didn't even know he was there
It was really really hard for both of us to come to terms with it but we knew for her it had to be done. Please PM me anytime if you need someone to talk to. I do agree that you will know when it's time and just remember you are doing the best thing for Willow. (((HUGS)))) to you, your family and sweet Willow.