I was the same when my parents had to put our old dog to sleep.. we had him since I was three... he was 15 when he was put down.
I think this was harder on my hubby than it was on me as she was his dog. She tolerated me I think... although I still was very attached to her as we had her since she was 10 weeks old.
But she let me know in no uncertain terms that it was time... no matter what day it was. To be honest, I didn't think about what day it was until after I was home from the vet and my hubby and I were talking about her.
I held her in my arms as she passed and I felt her relax and felt the pain leave her.... she's not suffering anymore though so that's what I keep telling myself to relieve my sadness.... although it doesn't work too well. *sighs*
Show me your horse and I will tell you what you are. - English Proverb