I've re-homed over 60 kittens/cats over the past 18 months and everyone of them going causes me tears, but you just have to think to yourself that without your help they would not have made it to a new home so we should all really celebrate - these kitties are all the lucky ones!!
You getting upset shows how much love and care you have put into them, what is wrong with that. Kelly you have done a wonderful thing and now you will have room to do it all again for another fortunate little fur baby.