I am sorry for your situation.  The lease is a contract as between you and the landlord.  If you both signed the lease then you are both responsible for the payment of the rent.  The only way for your child's mother to be released from the lease - or you for that matter - would be if the landlord agreed and prepared a new lease with only one of you on it as paying the rent to him.  If he does not agree then you and she remain liable for the rent, no matter who stays and who goes.  So if you miss the rent or "default" the landlord can come after wither and both of you for the money.  Good luck.