My failure as a foster mom

Our FD had shown me a completely different side of life. I have been filled with pity, compassion and affection for her as she has processed through her experiences. Her behaviors have been challenging in many ways and yet she can come SO far in the last 8 months. Many times I have been amazed that a child who has suffered such horrendous abuse has really perservered to such a degree. and then, yesterday.... She deliberately and with planning and with taking steps to avoid being caught, harmed one of our cats. This cat who has never done anything but love on her, every day. I simply cannot get past it. I don't want to even look at her. All my feelings for her are just gone, all I feel is anger. I am completely beside myself. This is not even any appropriate consequence (ok she's never allowed to even touch one of the cats again. whoopie.). All that is left is punishment and that's not appropriate either. I realize intellectually that it is not her "fault". That simply doesn't have any impact on how I feel. The kicker is that today is her birthday. So, she does one of the most unforgivable things one can do (only one step removed from harming a child, IMO) and today we get to spend making the whole day special for her. I don't even want to be here today and I know I am the worst mom in the world, no, the worst person in the world. Thanks for listening to me...I know some of you might understand and give me some words of wisdome how to find a way to turn this around.