It is a horrible thing with which to be faced. A pet is a member of the family, no question about it. I'm so sorry you have to make this decision. You just know when the time is right and it sounds like you have given this a lot of thought. I'm so sorry. It takes time to heal.
There is a lengthy thread on OMNI on death of a pet. Maybe you might want to take a look over there so you can see that you aren't alone with these feelings.
Bobette