Why does my Persian sound so 'robotic' compared to my Iranian aunt?

I’m a heritage learner who can understand most family conversations, but every time I respond, my parents tell me I sound like a textbook. I think I'm overusing formal grammar and not catching the 'shekasteh' (colloquial) contractions correctly. How do I bridge the gap between the formal Farsi I studied in books and the way my family actually speaks at home?