Do native speakers of your primary language tend to have a solid grasp of the grammar rules?

I'm a native English speaker in the US and have always wondered whether native speakers of other languages struggle as much as English-speaking Americans do with the grammar rules of their respective primary languages. For example, it's very common for Americans to use the wrong form of "your/you're" and "there/their/they're."

Edit: I agree that the example I gave could be considered more of a spelling error so I'll add the example of using "good" instead of "well" (i.e., saying, "I'm doing good" when the correct grammar would be, "I'm doing well."). That said, I do think the incorrect use of your vs. you're could arguably be considered a grammar mishap because it's related to word definitions and understanding of contractions vs. strictly spelling.

So, what about the native speakers of your primary language or in your country (I'm curious if this is the case in other English-speaking populations too)? Is it common for people to regularly make errors in their grammar, spelling, sentence structure, etc.? What sorts of errors are most common?