I know this is not an easy decision, but you know it is the right one. Yes, that doesn't make it less difficult to deal with.
My mom has gone through this with 4 dogs in the past 3 years or so. And each time is as hard as the one before.
Anna will be a happy puppy up at the bridge with all the rest of the pets waiting for that one fine day.
Hugs to you and your family.