Your situation is heartbreaking.

I have been through it before with my dog and 2 cats over the years. It was a very hard decision to make each time. With one of my cats I actually stayed with her and petted her until she was gone (the vet allowed me to). She never appeared to be in any discomfort whatsoever. It was all very peaceful and quiet.

I feel for you, may you find peace in whatever you decide.