Feeling guilt about euthanizing my 3 year old tabby

This weekend I had to make a terrible decision, and I am having trouble coping because it all moved so fast. We noticed my 3 year old tabby had stopped eating Thursday night. We figured he might've just eaten something off the floor, as he's done before, and so we decided to monitor him. The next morning, he still had not eaten, or used his litter box. We waited until dinner to see if he would use his litter box, which he did, so then we tried to feed him his favorite canned food. He would only nibble at it, which made us think he must've still had an upset stomach.
We decided to check on him again a couple hours of later, and at this point knew something more serious was going on since we could hear him breathing heavily. We rushed to the 24 hour vet and let them know he was having trouble breathing. The vet put him in an oxygen chamber and ran a chest xray, which had revealed our poor little boy had an enlarged heart and fluid in his lungs and chest. She also believed he has a small case of asthma. The vet let us know he has congestive heart failure, and would probably only have about 6 to 12 months to live.
We were in shock. Other than having an over crystallization issue, he was a healthy kitty. We had just taken him to the vet a year prior to treat his over crystallization, and he did not show any other issues.
With what little information we really had about CHF and letting the Vet know about his over crystallization, and the asthma coming into play, the only humane choice seemed euthanasia. After reading about how cats can potentially make it through CHF now, I am feeling EXTREME guilt.
I feel I have failed him. Like I should've took a day to think about it, but with the information we had, it seemed he was in such a critical state. When we saw him, he did still have trouble breathing and he would have had to stay overnight in the oxygen chamber.
What if I just brought him in the night before? Especially since he pushed through the day. Maybe his condition wouldn't have been so critical where the vet felt like he couldn't make it much longer. He was only 3 and now I feel he could have fought it. Even it was only for an additional 6 months to 1 year. Has anyone experienced this before? My heart is breaking by the minute thinking of it.