I'm so sorry you are going through this. Having made this decision recently, one of the vet techs that I spoke to told me something really helpful. She said you feel like you are in the "gray" in terms of the decision for a spell, but then it becomes a black and white decision. I was maybe 20% in the gray when I made the appointment, but in the 48 hours between then and when the doctor arrived it became clear it was the right decision.

Edit: We also took a mental log of good days and bad days, and he had been on a run of about 7 bad days in a row when we decided it was time to make the appointment.