Rescue groups are usually volunteers, giving up their time to rehome pets. Sounds like they put a lot of work into this for you to find a home for Cosmo. You had him for 13 months, until you decided you didn't want him any more. Why didn't you get him desexed in that time? I am sure he is desexed by now as that would have been part of the adoption process.
Pets are not objects to be handed around to the next person. I suggest you should have put a lot more thought into whether you wanted to rehome him before asking for a rescue group to assist you. The new family doesn't need you harrassing them about you wanting their pet. I am sure this didn't happen suddenly so that you had no time to think about it.
Better for you if you move on. You rehomed him thinking you were doing the right thing by him. You did. I am sure Cosmo is now perfectly happy with his new family who I am sure loves him very much. Hopefully they've groomed him so that he can see, looks like he badly needs a groom in some of those photos of him!
I am sorry, but I don't buy the "he was a cherished member of our family" and we now miss him. You should have thought about that while you had the cherished member of your family with you.
