Berlin has a lot of jazz, and a lot of sessions. Some do it regularly, every week, and others do it sometimes. Check out the sites to see when they are.
There are also some facebook groups which you can join to get all the current info on ongoing sessions: Jam session Berlin or Musicians in Berlin and so on.
Upgrade your session playing with new songs! A great way to learn new pieces is just to listen.
That's why I created this random jazz standards page that selects songs out of a list of 1300 standards with YouTube video's: Random jazz standards