I am so sorry for your loss
I know exactly what you are going thru I had to do this 2 times over the years and it never gets any easier, it feels like your heart is ripped right out, and yes Perfect word for it you feel Drained.
And second guessing yourself is normal I did it too, I felt like "I'm not God, How can I choose when a pet should die! Well as it was I had to do it, the first one had very bad Mouth cancer and she could no longer eat, and the second one had feline leukiemia and could no longer live the way a cat should, she was having severe breathing problems.
Again I am So Sorry for your Loss