It is hard. I've only given up one foster so far, but have another two here who will probably go at some point (depending on how long Summer lives). I think, for me, I have to look at it like its the ultimate act of love, because I am doing what is best for them. Sometimes PTS is what is best for a bunny, and its what is best for them, even if not for me, but with fostering, the best thing is to have that forever home, and whilst it is hard, its the best thing. That makes it easier for me, personally. Sure, I will miss them, and I desperately missed Harley, but it was very bittersweet, bitter for me, sweet for the bun, which I found really helped.
I do think though, whenever you 'lose' a bun and they stp living with you, you do have to go through a grieving process with it.
HOPEFULLY though, Snowball will find an owner who will keep in touch with you, because then you can get updates and will know you did the right thing