Update on my little old man

Posted my boy here a while ago and I have some news, his 15 by the way, for a recap or those who didn't see my other post and the comments, my old man had an infection in his eye, I was very distraught over the thought of him being sick and possibly loosing him, I took him to the vet and they looked at his eye, it was very inflamed and after a week of anti inflammatory, anti biotic and topical pain relief he was getting better, the anti inflammatory helped his arthritis as well and he's so much more lively he had a follow up the next week, they suspected glaucoma, he was put on new meds (a new topical pain relief, anti biotic and anti inflammatory) and another follow up scheduled, he just had that follow up today and I'm so torn up about what to do. She (the vet nurse/surgeon) says the medication is keeping the pain at bay, but it's going to get worse, I was quoted $3k (Australian) for an eye removal and teeth (he has gum disease, has for years) I cant afford that, not now, not in the coming months. I'm not even sure if he will go through surgery, my family is telling me just to let him rest, and I'm okay with that but I don't want to feel like there was more I could do. I need to come to terms with the fact that this is his last Christmas, and I want him to at least finish this year. Putting him to sleep is going to be cheaper then the surgery and then I know his at rest, I won't have to worry about his pain anymore. But he's been in my life for 15 years and I don't know if I'll be okay without him, after loosing my grandparents within a year of each other and now little speedy, idk how to live with the constant grief and sadness. But I know it will happen one day regardless and I need to be okay and move forward. I also want to give family enough time to say their goodbyes. He was loved by everyone.