I am so sorry about what you are going through. It's always agony when they go, and very very hard beyond words, when we have to decide to have them put to sleep. It goes against all of our instincts. But when we know they are suffering, and it's the only way, then we do that deed with love and compassion.
There is always some guilt about something we didn't do right. I know. When I had my Misty put to sleep, I honestly had tried everything, and was sure I did not have even one regret....but in the weeks to come I surely found something I wished I'd done differently....and then another thing....and so on. I found myself remembering all the little details of our life, and maybe the things I did wrong over the years...
But the truth is they loved us, and their lives with us were OK. They loved their lives, and loved being with us, and wouldn't have changed it for the world.
You have my heartfelt thoughts and prayers X