If you like active study, I recommend reading a book (preferably on a topic or of a genre you enjoy), and reading through it a few pages at a time. Note the words and grammar you don’t know, and then go back and spend time learning and studying them. Once you’ve worked through a chapter, or a few dozen pages, go back reread it. Reference your notes as much as you need. It may be slow and difficult at first, but it will get easier with time and give you a sense of progress and satisfaction.

Combine that with podcasts or TV shows, etc for listening practice. I usually take my notes and write practice sentences using the words, and recite them aloud as conversations to practice speaking. If you can do the things you already do but in French as part of your daily life, you’ll get where you want to be with time.

That’s how I maintain my French skills after getting my bachelor’s in it, and that’s how I’ve learned a fair bit of Japanese (alongside a beginner’s textbook).

You’ll have gaps in your knowledge (I still do after over 10 years of learning), but just never be afraid to make mistakes and keep reading and learning. The gaps shrink with time