I'm so sorry you're going through this, and my thoughts and prayers are with you.
I've lived with many animals through many years of life and understand that euthanasia isn't easy. It wasn't a choice immediately in front of me many times for at least a couple reasons. One is that I grew up on a farm, and many pets died in different, unpleasant ways because I wasn't allowed to have many cats, dogs, etc. in the house for any length of time. Another reason is that our own kids had many small animals, i.e. gerbil, hamster, birds, turtle, fish, etc. that died rather quickly.
After being married for several years, we faced the decision with our 15-year-old dog. I took the dog to the vet very soon after I noticed that he seemed to be in pain. The vet diagnosed cataracts, a liver problem, and more. I asked the vet to tell me why I should do it. (I think I knew--but it was difficult.) He talked me through it, and my husband said he would make the decision for me. I've never regretted it vs. letting the poor animal be hooked up to tubes and in a vegetative state--a poor quality of life. Another much younger family member went through it twice in a short period of time with two young cats.
I'm omitting many of the details here, but there were several trips, treatments, hope of some improvement, sad feelings of the whole family--and more.
I think one usually has a fairly good idea at some point whether it is the best choice. One of our own young cats got better within a few days after the vet didn't show much hope and had trouble diagnosing the problem. I personally just don't support a poor quality of life or pain for an animal but also want to be as comfortable as possible with the diagnosis and the message that everything (within reason) has been tried. Every situation is probably unique.
God bless Annie and you as you work through this.