I know what it feels like and I wish you strength and peace in these hard days
I had to take it day by day, otherwise I found myself literally making myself ill with stress.
Treasure each day, enjoy as many of the fun things as you can without pushing her too much. Talk to the vet about pain management, with probably weeks at most left you don't have to worry about side effects and they should be able to help you with a management plan to keep her comfortable.
In the end Alex told me it was time, on the last night he was whining in his sleep which he never did and he was very mopey and miserable from the pain. I knew it was time and made the appointment for that evening, one of the hardest things I've ever done but I stayed with him for every moment. Once the sedative took effect he was so calm and peaceful for the first time in days and I knew I was doing the right thing. He snuggled into my arms and sighed out his last breath. Knowing he was free of pain and happy again was worth every tear I shed and I truly believe I'll see him again so that is a comfort
The only thing I'll stress about the actual process is insist
and don't back down on giving the sedative beforehand. It really makes the whole process smoother and easier