I immediately had the landlord draw up a new lease with my name on it only.
I'm sorry to be the bearer of bad tidings, but you went about things backwards and now you're stuck.

What you SHOULD have done was to seek out a new roommate, then take your old roommate to court for her share of the rent in the time between when she moved out and when the new roommate moved in.

By immediately having a new lease drawn up, you essentially consented to her breaking the contract, and you willingly took on the responsibility for the entire rent payment. I suspect that if she sues you, she'll prevail.