I want to go away without my step daughter
Why do you want to go away without your step-daughter?
I just don't want to live with her anymore, I'll just go somewhere else
You should not exclude your step-daughter from your life. You are with her parent, so they both are a part of your life together.
How do I tell my partner I don't want the step-daughter around?
That is not something you should say to your partner. You need to learn to live with her as best you can. Your partner will appreciate that.