How do you know when its time to put your dog down?

I have an 11 year old Brittney Spaniel with and enlarged heart and fluid in her lungs. Is it time to put her down? Shes not eating either and she wont walk. I have to carry her outside to use the restroom. This is the hardest decision i've had to make in my life. Shes been in my childrens entire life and i just can't choose which is best for the dog. I really don't want to put her down, but it may be time.