Really sorry for your loss. It sounds like you made the decision based on what's best for your fish. That's not an easy choice to make, but it is the kindest thing you can do sometimes.
I felt awful when I euthanized my goldfish earlier this year, and even worse when I stupidly held on to hope that my big Oscar would pull through after he started going downhill. I wish I had given him a peaceful exit, instead of allowing him to suffer until he finally died.
It's just a part of life, for better or worse.