I came to the conclusion a long time ago that I could never become truly "bi-lingual" unless I managed to forget about half of the English that I know. I am reasonable comfortable carrying on a conversation (or reading books) in Spanish (and when I was in better practice, in French, as well). By some definitions (not mine) that would make me 'fluent' (though of less than "native proficiency").
What did it for me was looking around the kitchen at all the cooking implements (when I was, perhaps, in my twenties) and realizing that in English I knew such words as 'colander', pressure cooker', 'garlic press', 'double boiler', 'spatula', 'egg-beater' and a host of other terms. With a bit of reflection, I added the names of dozens of tools (hardware items) that I had learned as a child. I include in the list "sash weight" (a large chunk of iron that facilitated opening and closing old-fashioned windows) It occurred to me that I had learned all of these words as a child but had no idea how to say them in Spanish or French. It further occurred to me that I had never heard/read them in Spanish or French and that, in all probability, I never would. Many of these are words that I might have occasion to use once in 5-10 years in English but they're there, tucked away in the back of my brain, relics of my childhood.
Professor Higgens of "My Fair Lady' fame (or, if you prefer, the corresponding character in Shaw's "Pygmalion") astounds his listeners by correctly predicting (on the basis of a few utterances) that a certain person was born in X part of London, but after the age of 10 (or some such) was raised in some other part (Y) of London and, as an adult lived in region Z of London. There is an underlying factual basis for this sort of thing. Studies have shown that certain words (or classes of words) are typically learned at various definite stages of life. That is to say, there are a bunch of words that you probably learned between, say, the ages of 5-10, others that you learned as a teenager and, still others, that you learned as an adult. The fact that you learned a particular pronunciation (or expression) can be predictive of where you learned it (which, in turn, suggests where you were living at that time in your life).
Even the term "native speaker" poses problems (at least, when it is interpreted as meaning some sort of "expert" in his native language). Mostly, the term should suggest a competence in recognizing "grammatical"/"ungrammatical" utterances. When it come to issues of vocabulary, the situation changes completely. The skilled carpenter/plumber/car mechanic does not know/recognize the specialized vocabulary of the brain surgeon and neither understands the specialized vocabulary of the astrophysicist (nor he, their vocabulary). The astrophysicist may not be able to explain the difference between an 'adjective' and an 'adverb'. They each have expertise (in their respective areas) but this expertise does not qualify them as experts in the language.