Can’t ride japanese bikes?

So this seems to be a unique problem.
Back in the states I rode bikes all the time as a kid/teen, I was pretty good at it and absolutely loved it. Didn't ride for a couple years then moved to japan back in 2021. Long story short I went to Fuji with some friends who wanted to rent bikes and I thought it sounded fun, only to realize when we got them I couldnt balance or steer that thing for shit. It was absolutely mortifying and she kept asking why I didn't say I couldnt ride a bike – i literally always could?

Found out later its most likely because theyre city bikes vs mountain bikes – the bikes I remember riding were so light you could lift them with one finger, wayyy faster and much more comfortable for me, they had to have been mountain bikes. But now its been almost 4 years in japan and I want to be able to bike because the trains are expensive and not great for local travel, but I have no idea how to go about learning to ride a city bike without annoying my neighbors or being a menace on the sidewalks. Chances of my finding a mountain bike here are slim to none i know, and im not even certain if they're allowed on the roads – any advice?

Ive also debated trying a motorbike – the balance itself isn't my issue as i can balance a mountain bike and even a city bike just fine, its the balance + peddling action with the weight of city bikes (plus how slow they are to get started makes them a nightmare to keep balance on for me since I learned with a bike that almost immediately has momentum going to help you keep balance). Would that be worth considering since it would technically take out the peddling/starting up part that throws my balance off?

I was thinking to try out with one of the types you dont need a license for at first. Does anyone have experience or advice on how i could try any of these options? Do they have adult bicycle classes here or anything? Or do i just have to pray for the best 🫠 i know some of us might have also come from a place that doesnt really have city bikes so im guessing im not the only person in the county who can't use them as an adult, but it seems few and far between.