Why do the verb endings feel so different when talking to family?

I grew up hearing bits of Sinhala at home, but every time I try to use the grammar rules I learned in my online course, my relatives look at me like I’m speaking a different language. It’s the diglossia—the formal vs. spoken gap—that’s killing me. How do I bridge the gap between textbook Sinhala and the natural, colloquial dialect used in daily family life?