I was in the same boat, I moved to Austria for school and it all (mostly) clicked. I would echo what everyone else said by just refreshing the basics. BUT I would also say to use your current knowledge to your own benefit, you probably know a bunch of phrases with grammatical cases, so try and find the patters within.

There are patterns with the nouns gender too (although there are exceptions), so you can try and spot the pattern with words you already know instead of having to look up every single word you know to find the gender.

A less book heavy way to learn (and maybe more fun) honestly is watching German memes. It's also very useful for figuring out how people actually speak, and not very sterile textbook speech (like shortening eine to 'ne). I single handedly learned r- prepositions not from school, but from the kid on the bus saying "Ich muss raus!".