French Student (intern) - Looking for a 5-months flat - April-August

My name is Antoine SZATKOWNIK, I'm currently studying bioinformatics at Sorbonne University in Paris.
In the course of my Master 2, I have to do a 6 months internship, I did found one at the Robert Koch Institute in the Bioinformatics Department.
I'm 24 years old, I'm serious and I'm looking for a nice place to stay and enjoy my week-ends.

For the past few months I've been coming to Berlin because my girlfriend is currently doing her own internship here. Until march 31st, I will be living with her until she moves back to Paris, that's why I'm currently looking for a place from the 1st of April to August (it can be few months or the entire period)
I want a place where my girlfriend will be welcome to come visit me too.

I'm entirely vaccinated ;)

Thank you in advance for you interest in my request
Cheers

Antoine