Being a step mom

So I’ve been with my man for two going on 3 years. He has a 9 year old daughter that I’ve known sense she was 7 And we’ve had our ups and downs but we always pull through. But the past few months we have been struggling to see eye to eye about things and our little girl is acting up about me being pregnant. I don’t really know how to react and I’m honestly just trying to not think or stress about it. But I know that won’t help the situation. 😞 anyone have a similar situation that they could give me some advice? I’d appreciate any help 🖤