Euthanasia for older cat.

I euthanized my cat on Friday, and I regret it so badly. He was 14 1/2 years old, blind, and had diabetes for 5 or 6 years, which was controlled by daily insulin shots. He also had feline aids. Occasionally there would be an issue which would cause a dosage increase or decrease, but then he would be fine. In 2011, he began losing hair in chunks, and losing weight. The hair around his head and shoulders grew back beautifully, but the rest of his body (not including tail and legs) was pretty sparse. He walked slowly but we assumed that was due to his lack of sight. He still slept on the bed and used the litter box. A couple weeks ago I took him to the vet because he was vomiting and had diarrhea. They gave him antibiotics and injected fluids under his skin. He had no fever. The vomiting stopped, but the diarrhea continued. Upon weighing him, he was down to 6 1/2 pounds. He spent most of the day sleeping, and wasn't eating or drinking much. We did full blood work, and it all came back fine except for his pancreatic enzyme was up. The vet initially though pancreatitis and said we had to wait through it. This past week I took him in every day for IV fluids. He still wouldn’t eat more than a teaspoon of food, and wasn’t drinking anything. Starting Wednesday, he stopped using the litter box, and would usually lay where I put him, except for occasionally urinating or defecating diarrhea on the floor beside the litter box, or in my bathtub. Thursday while at the vet for IV fluids, he had a seizure. Since he wasn’t eating, his insulin dosage was too high causing his blood sugar to drop. The x-ray taken at the beginning of the week didn’t show anything. His diarrhea still had some chunks in it, and the vet couldn’t feel masses. He was trying various antibiotics. The vet finally said that there wasn’t anything else he could do, he was sure he was in pain. He suggested exploratory surgery, but when I asked him what he thought he would find, he didn’t have an answer. Through all of this, my sweet cat Midas would purr whenever I pet him. I asked if I was prolonging the inevitable and causing him to suffer when he wasn’t going to get better, and the vet said yes. He then said that they have tried exploratory surgery, and if nothing was found, they proceeded with euthanasia without waking him up. I made the decision to proceed with euthanasia so Friday morning, I took my best friend in and had him put to sleep. I’ve never regretted a decision so badly in my life. Why didn’t I proceed with the surgery? I have no excuse other than I was worrying about paying for it, but he so deserved for me to do whatever I had to do to try. I can’t go back, but I hate myself for it. The vet even said that it’s unusual for a cat to survive as well as he did with the diabetes, but I can’t get past my terrible decision. Was I wrong?

Thank you, I needed to hear that from someone impartial.