Hello Roj.
I found it very helpful looking around some groups on facebook and sites as well, where people post their free places, and you can also ask what you need.
Facebook groups:
https://***.facebook.***/groups/wg.wohnung.berlin/
https://***.facebook.***/groups/wg.wohnungen.berlin/
Here are some of the sites:
https://***.studenten-wg.de/wohnungsmarkt.html
You can search for rooms or shared flats - and you don't have to register or pay.
Maybe you'll find some interesting offers on these websites as well:
****://***.wg-gesucht.de/1-zimmer-wohnungen-in-Berlin.8.1.1.0.html
****://***.wohnungssuche-berlin.net
https://berlin.homecompany.de/de/index ->
https://berlin.homecompany.de/de/suchergebnis/searchpreset/waz/1-zimmer-wohnung
****://***.exberlinerflatrentals.***
Good luck!
Let us know if you find a way.