I grew up in London and I’ve lived and worked abroad for a couple of multi-national countries. While I’ve met literally hundreds of non-native English speakers who speak excellent fluent English, the number of those whom I would consider totally fluent to a native level (in other words, as proficient in English as myself) is maybe five. It’s really rare and extremely impressive.
These people may have an accent, but their speech is 100% native-level with none of the small giveaways that they didn’t grow up speaking it. Based solely on your written English you’re probably 95% there especially given your age, but that last 5% will take years. If it’s something you’re really serious about doing then there’s no substitute for living in an English-speaking country for a short period.
NB: I’d still consider you “a fluent English speaker” if your spoken English is as good as your written English, so don’t take this the wrong way! It’s just that getting beyond “fluent” to “perfect/native-level” is a big task.
Maybe we should change this thread title to “English advice for foreigners”?