Yeah, I really struggle with the concept of "native" as well. I grew up in the US but the first language I spoke was Tamil. When I went to a nursery, I couldn't communicate with any of the other kids. My parents then spoke to me almost exclusively in English. As a result I can only speak/read/write English but can still understand Tamil at an elementary level.
I used to think I spoke perfect American English until someone pointed out that I say the th sound weirdly (I think it's called dental th-stopping [0]). I also spent some time in England as a child and now say some words like "rather" with the English pronunciation. I'm living in England again now as an adult and am picking up some English colloquialisms, though usually pronounced in an American accent.
I also unintentionally end up speaking in an Indian accent when I talk to speakers of Indian English, but can't put on (even intentionally) an English accent to save my life even though I had one when I lived in England as a child.
[0] https://en.wikipedia.org/wiki/Th-stopping