I am so sorry for the loss of your precious Cosmo
You have just went through what we all dread and what a lot of us have gone through. I know that it will take time to be able to remember Cosmo and all the wonderful memories you have of him as of right now you are still in shock over this sudden decision you had to make.
I think each of us is different and we each grieve in our own way. What is right for one, may not be right for another. You will have to follow your heart and know that you could never replace Cosmo but he wouldn't want you to be lonely and sad. I pray that your heart will heal in time and you can give another furbaby a place in your loving home.