Hi----I appreciate your problem : I was hopeless at French A Level at school, my worst A level subject.
When we moved to France 4 years ago, all my old memories of the grammar and sentence structure came back ; and vocabulary came faster than I had imagined.
My wife spoke no French at all, not even at school.
The brief answer to your question , in our humble opinions , is : move to France and if you mix with locals daily you'll be fluent within 6 months if you've had any A Level experience; and within 12 months if you've never learned any French previously.
The best way to master the language----the ONLY way-----is to live in France full-time.
And isn't it a beautiful language ?
Wish we could have moved to La Belle France earlier in life. But better late than never.
Bon courage !