As has already been said you will be beating yourself with a big stick right now. I was exactly the same week ago today when we pts our 13 year old Golden Retriever, Spike.
For the first few days I thought about nothing else and thought I would go mad. I kept playing the scenario over and over in my head, I couldn’t eat (and felt guilty if I did) I couldn’t sleep and I just felt so empty. These and more are all feelings that you will probably experience. It is all part of grieving and if we didn’t love them so much we wouldn’t have these feelings.
When you doubt your decision, please try to balance this by reminding yourself that you did do the right thing for Gia - even though you knew it would be you who would be left behind to suffer her loss. That is true love and devotion.