I left my 2 girls at a cattery for the first time in March, when they arrived they hopped out of the baskets and went and sat in the upstairs but of the unit which had a little window that looked out to where my car was parked, as I walked back I could see 2 little faces watching me go, I was crying most of the drive home as I was in the car on my own with no OH to comfort me.
When I got home, he assured me that they'd be fine and sure enough when I got back the boy that helps out there was telling me about how affectionate they are and how much they loved cuddles. They weren't too keen on getting back in the baskets so they must have been spoilt!
I think the first time must be the worst, at least in my head anyway as I think once its happened to them once and they've been taken home again, hopefully they will know in future that its not forever and you're not abandoning them. Mine were rescues as well which made me feel doubly bad.