I'm dating this guy who has a daughter. She resents the hell out of me. I'm always nice to her but she disrespects me a lot, especially when he's not around. I've tried to be the adult but she's testing my limits. Absolutely this could interfere with our relationship. ?????
This is very common. She looks at it like you are stealing her father, so she's fighting you for him. Get some professional help with someone who has experience with this. It will make life much easier.