I want to relearn a language I already kinda know. How should I go about doing it, Reddit?

My mom is from Japan and my dad is American. I was born in the US and started learning Japanese when I was a kid. I went to a supplementary school on Saturdays that followed the curriculum of Japanese schools, and in the summers I went to Japan and studied the language there.

Not surprisingly, juggling both American school and Japanese school, going to school year-round on two different continents, and trying to keep up, was very difficult for me to handle. When I was around 12, I convinced my parents that I could only concentrate on American schooling, and they thought I had a strong enough foundation in Japanese to quit going to supplementary school.

So I have perfect pronunciation, can read hiragana and katakana, and have something like a 5th grade level for kanji. I'm a few years shy of 30 now and was wondering if there would be a way to pick the language up again, and further my education to maybe a high-school level. It's pretty embarrassing when I start speaking to my Japanese family and they have to dumb down everything because I only understand up to a middle-school level of vocabulary.

Worse still, I feel like I'm regressing and without practice I won't be able to read or even write anymore.

I know there are things like immersion or Rosetta Stone, but I was thinking of something more thorough than that. Reddit, can you help?

TL;DR - Used to know Japanese like a native, stopped studying it in middle school, now wants to pick up where I left off before I lose it altogether