I feel like a fake speaker when talking to my Korean relatives

I grew up hearing Korean at home, but I never mastered the honorifics (존댓말). Now that I'm trying to actually speak it with my grandparents, I feel super awkward because I fluctuate between casual and formal forms, which probably sounds terrible. How do I bridge the gap between 'heritage listening' and actually producing natural, respectful Korean without sounding like I'm reading from a textbook?