Aw Sarah, i understand completly.
I've gone through several hundred of my foster kittens getting adopted. It never really gets any easier- but you do get more at peace with it most of the time when you see them go to an awesome forever home.
If you kept all of them, you wouldn't be able to help any more needy kittens that come your way -and i just think of how many little ones will need you down the road!! There's always a sweet face in need of a good foster home!
You do wonderful work hon - and you definitely saved those little ones lives! What better joy than to see them go to a great new home?
It'll be ok!!
(I do on occassion get super attached though- there's 4 that come to mind on a regular basis that i regret not keeping. I think about them almost daily: Bosco & Berkly -one seal point siamese and one himi. both i had as tiny kittens - they found awesome homes -but i was SOOO attached to them, it was hard to let them go. Jezabel, my 3 legged kitten - i miss her SOOOOOO much. I regret not keeping her! and Persia....the rescued 4lb persian that took me 8 hours to shave all the mats off of and clean up- the horrid shelter i used to work for put her down without telling me!
- they knew i was in the process of adopting her too!) I was furious. / So yeah, there are definitely ones that grow on you that you will miss- but it will be ok. Most of the time they go on to wonderful forever homes- and it sounds like you've done a great job of screening them too