Do you still have native-level pronunciation and grammar skills in Cantonese? Forgetting words is one thing, that happens, and words can be relearned. I’ve lived abroad for 5 years now in my 20s, and even I’ve lost some vocabulary in my native language.

If you no longer know the grammar or the pronunciation well, then it’s a more legitimate question if it is your native language anymore.

Either way, you are some sort of bilingual. In fact, some people grow up like that where I come from: with two native languages, one of which is oftentimes stronger / more eloquent due to education, social life, etc.