I’m a native English speaker who became fluent in French at age 33. How? With a French girlfriend who didn’t speak English. Yes, I was confined to the template of her particular brand of French, but after a few years I branched out, I read, I listened to French singers. Academic learning was a small part of my path, less than 1% I’d say. In the end my French became almost native, at least according to tests I’ve taken. I learned like a child, by imitation. My accent got good, but that’s an individual thing.