I really sympathise with you it’s devastating. We have recently lost 2 close together. The worst thing about having pets. No advise really other than remember the good times and cherish the memories.
I recently had to make the decision for my dog. I cried two days before, didnt cry on the day and never since, until I read an article about Monty Don (Gardener) losing his beloved dog and that let loose the tears big time.