Feel like I killed my best friend

I'm having a really hard time with the decision to euthanise my best friend Saturday. He was such an awesome, wonderful friend and my protector. The chances of him recovering were very slim and he was in pain and on top of that he was sick. I still think that if I kept him alive he might have recovered but now I'll never know. I just really miss him and feel like I made a terrible, selfish decision. I just want my buddy back