One party trying to break the lease?

My boyfriend an I signed a 6 month lease July 1st and I want out of it. I'm not sure if he will stay at the apartment or not, I really don't care. I just want out, it's too much of a pain living with him. His grandmother is a co-signer and I don't want anything to reflect badly on her if I break my end. So how would I go about doing this and how much would it cost me?