Dual citizenship (Brazilian/ Japanese)

Hi,
I've been researching on dual and multiple citizenship and still have a couple questions...
I'm 24 yrs old, Brazilian, and daughter of Japanese national and was told by my father I needed to do this selection of nationality. He mentioned when I was to turn 21 but didn't return to the subject. Therefore, I haven't done any choice of citizenship. As for what I believed, I still hold my double nationality, both Brazilian and Japanese.
I've read, and been told, that it wasn't an obligation to renounce citizenships.
As for now, according to Japanese law, I'm no longer Japanese national (child of Japanese national) and have no manner to reverse so?
Also, I'm not certain how it works though in Brazilian law. If I had chosen to retain my Japanese citizenship would I be considered a foreigner in my home country, being born in Brazilian territory and daughter of Brazilian mother? So confusing, doesn't make any sense to me.
I've asked many people I know, sons and daughters of Japanese nationals and in similar position but still no clarification. Can someone enlight me on that?

If you choose to hold your JP citizenship then you need to apply for the Brazilian permanent residency.
If you choose Brazilian citizenship, you can not renew your JP citizenship.
JP Govt does not allow dual/multi citizenships after 22 yrs old.

I'd start at the Japanese embassy or consulate in your country. They're the experts.

My suggestion, take up the Japanese citizenship & apply for a Brazilian PR (Permanent Resident). Being born in Brazil would increase your chances of getting PR.
Reverse is however very difficult.
Being a Brazilian citizen you would need proper visa to enter Japan, then continue living legally for several years in Japan only then you might be eligible to apply for Japanese PR.