I am so sorry you are faced with one of the hardest decisions that we as our pet's guardians are ever faced with. I agree with so many of the above when they say talk to your vet. She/he needs to completely understand what your concerns are (being away from Daisy during the day, pain management, signs and symptoms of exacerbation, etc.) Please make him sit down with you (not stand up behind a table) so that you have his total attention and he understands how important this is. Even the best of vets can sometimes forget how much this decision can change our lives forever.
The others talked about you knowing your pet better than anyone else and being able to read subtle signs and symptoms that others might miss. I will never forget the day I had to make "that" trip to the vet with my Gigi. I was beyond myself with self doubt but, even so, with her terminal diagnosis I could see she was becoming constantly tired; she was isolating herself from the others; and her eyes and her gait told me she was starting to feel pain, in spite of the medications that had held her so well previously. I truly believe, if we listen to them, they will tell us when. I wanted to be selfish and keep her longer but how could I, when I knew she had done nothing but give me her unconditional love all of her life. I had to take on the pain so she wouldn't. My heart aches for you - you are in a horrible position - but I do believe, you will know when. I pray for your Daisy and for you.
My warmest regards, Carmen's and Gigi's mom - alicia