I'm very sorry.Our dog has cancer and it’s progressing more rapidly than we thought, it’s moved to her lungs and breathing has become difficult.
I have an internet friend who once wrote an excellent post about how to decide when it's time to have a dog euthanized and I wish I could find it to link to it. she said it really well and I had an easier time with the decision that last time I had to make it after reading what she said.
the bottom line is, people worry about doing it "too soon" and instead we should worry about too late.
if a dog is suffering, that is the time. but you know this.