@Tris Prior I feel you. I'm going through some decision making with my own "only" 14 yo cat right now. Something that has put it in perspective was my experience with my first cat.
She was diagnosed with undifferentiated heart disease when she was around 6 or 7 (after a very bad reaction to steroids that were prescribed for an infection she had). She almost died then. The vets then told me she might live a full long life or she might die tomorrow, it was impossible to predict with her particular condition. She was put on a diuretic and a blood pressure medication to keep it under control (both low doses and cheap and easy to give to her).
Fast forward 7-8 years of perfectly healthy life when she started losing weight and not interested in eating. She had an ultrasound that revealed substantial thickening of her intestines that suggested either serious IBS or cancer. She had never shown signs of IBS before. She could not be on steroids (at all) because of the heart disease, and that's one of the first things they try with IBS. Tried all kinds of things. Appetite stimulants didn't do anything. Kept holding out hope that we would find something that would work. She lost a lot of weight and wouldn't eat. Didn't do a biopsy because if it was cancer it had spread all throughout her intestines already anyway so it would have been a useless diagnosis (so we focused on the IBS angle). I spent probably the last month of her life force feeding her hoping to keep her alive long enough to find a cure or something that would work to get her to eat again. It was awful and I regret it so much now. I didn't want her to die, it made me so sad and hurt so bad, she was my first and I felt like I had failed her. The truth is she had a great life with me for 14 years and it was just the last little bit of her life that was not good. I should have let her go sooner, but I didn't know any better at the time. I do still miss her and she has been gone for 2 years.
We humans are selfish. We don't want them to let them go because it hurts
us. But somewhere I recently read that cats and dogs don't have a sense of the future the way we do. We plan and think about the future and it saddens and angers us if we know that will be cut short. But they don't do that, they live in the
now, and aren't sad about not living in the future because they don't think about the future. They aren't afraid of dying, but the might be afraid of pain now. The best thing we can do for them is to give them a good quality of life now, and if we can't do that anymore, especially for older animals, it's not a tragedy to relieve them of their pain and suffering. It's ok to say "enough is enough".
My current 14 yo cat has IBS (definitively, and has been living with it for 3 or 4 years, successfully treated with medication). Over the past year she has become less interested in eating and we've had to up her dose and increase appetite stimulants to get to eat enough. She still lives a pretty "active life" (she sleeps a lot but still seeks cuddles, talks to us, gets mad at the other cats...). Her blood work is perfect (so it isn't something else). She's not super keen on wet food either, I try all sorts of things to entice her. Some things work some things don't. She has started to show some side effects of being on the prednisolone for so long. Her weight is way down from her all time high of 3 or 4 years ago. It is getting to the point though where I am not sure she isn't uncomfortable all the time, because if she was, it wouldn't be such a struggle to get her to eat. I don't want her to die either! Either way, though, the prednisolone will eventually kill her if she's on it long enough, and she can't not be on it. I think I am going to have to make a hard decision for her sake in the near future. I know that for her, too, she had the best life I could give her.
Anyway, just some food for though, and hopefully not to make you sad or feel guilty. Sometimes some perspective is good. Just know that you have done right by her so far, and she has lived a long life, and when the time comes to make the decision it isn't anything to feel bad or guilty about (even though you undoubtedly will).