I remember when the Shetland Sheepdog I grew up with had to be put down. It's one of the most painful memories I possess, but at the same time I know it was for the best. She had some serious illnesses, was in a lot of pain, and could barely get to her feet anymore. As much as I miss her, I feel better knowing that we allowed her to go peacefully before she got any worse.
It really is a hard decision to make, and I wish you didn't have to make it. There's sites and support groups available to help with pet loss, if you feel you need it, and of course you have the love and support of all of us here at Eden.