So moving to another country and setting up your life so that you have no choice but to speak a new language, is a guaranteed way to become at least conversationally fluent.
To keep the language, you need to keep practicing. For example, staying in touch with your new made friends helps a lot. Although, they are forced to read your letters out loud, in order to understand what sounds you’ve tried to put into words like a coded spy message. Ups.
Make sure they are nice and patient people.
Writing in French still, doesn’t make sense to me. Neither does English, but in case of latter, I had a teacher who made me memorize everything from random texts to grammar for the longest 12 years of my life. Every day. Every year started with conjugating every possible verb out there. It is the worst possible way to learn a language and cannot be done when you’re relying on your motivation alone. School sucks.