Help with my step daughter.

I feel so awful. My oldest step daughter has been behaving horribly for over a month. She hadn't been home all week until today...her birthday, she is only here to get stuff. And I'm so cross about it.

She needs stood up to and the behaviour clamped down on. Her mother doesn't bother and my partner doesn't do much for fear of pushing her away. I try but often the response is that I'm not her parent. At the moment I hate who she is turning into and I hate myself for feeling that way. She has the potential to be wonderful and it's horrible to see it disappearing. I don't know what to do anymore.

Now I'm pretending to be happy (and celebrate her birthday) when I feel anything but.

Any advice would be welcome.