I can completely empathize with you... I know how hard it is.
My purebred Black Labrador was only 11 when he started losing weight drastically and had trouble with his bowel movements.
He died with his head in my lap... It was the hardest thing I've ever had to do. I was 15 (now 19) when he was put to sleep, and my mom STILL beats herself up, wondering if she made the right decision.
I'm so sorry for your loss.