Sounds like Cosmo had a great life with you and your family. When we had to put our little Pippin to sleep at 13 y.o., it was the hardest thing we ever did, but it was for the best, just like the discission you made for your little pal. I think about my first Yorkie at least once a day and will never forget him, and that is what I'm sure it will be like for you and Cosmo. We have 2 Yorkies now and they dont replace him, but we love them just as much.
Hope you learn to cope with your loss soon as we did.