Moosmom, don't let yourself feel like that. Medussa is right, I also think somehow you will be repaid.
Chritmas is a tough time for lots of people but you should feel happy that your good deed did in fact help your friend. Her relationship isn't of much importance compared to the good deed.
I can't count the times I've gone out on a limb for someone and felt I had that tag "sucker" pasted to my forehead. Then I remember how good I felt for helping and what the future might hold and I toss away the feelings of being taken advantage of. Things tend to even out in the end when it really counts.
The kittie sounds like it might have just been called to a better place , maybe for the best and to save it from more suffering so don't lay a guilt trip on yourself over this.