If you both signed the lease, then you should inform your landlord on what's going on. You said "flat" instead of apartment, so I'm guessing you're not in the US. Here we could take someone to small claims court, but you need to start with your landlord. See if you can get out of the lease. If not, you will have to find a roommate or get a 2nd job to help with expenses.
Even if you take him to small claims court, and if you won, they won't help you collect the money from him. That's your responsibility. So you could "technically" win but the reality is still the same. Good luck.
