I do feel your pain and have been there
It does take time for the heart to stop feeling like it's breaking in half, and eventually you will be able to think of your Cosmo and smile and laugh at the funny things he used to do
When my Starr died, my husband and I just laid in our bed and bawled all night long, we were devastated. I said I never wanted another dog EVER, but DH insisted that we get another right away. I was so mad at him for pushing another puppy on me the very next day
She was also sickly and almost died, but you know what? It really did help us to heal, having this new little life to nurture. Luna (and then Izzy) did not replace Starr, not at all. Our hearts just expanded to create their very own spaces in it
We still talk about Starr, look at pictures of her, and remember her antics with fondness and love.
Only YOU will know when/if the time is right to give another furbaby your love. Let your heart be your guide
RIP Cosmo