I agreed to co-sign a one year lease on an apartment for my girlfriend. Now, we are no longer together. Is there any way to get me out of being the co-signer?

No, you will be obligated to pay the bill of she does not for the entire term of the lease. You will need to notify them that at the end of the lease you will not be responsible for any renewal of the lease.

You can ask the landlord to release you, but do you really think he will? As Chuck said, just make sure you inform them you will not sign a renewal.