Legal Question in Civil Litigation in Florida

It is a breach of contract. Hopefully you have something in writing from her making the promise.

First, tell the landlord of this scenario and let them know whether you plan to pay the full amount by yourself until you can find a new roommate. Start attempting to mitigating your damages (bringing in a new roommate). That roommate will likely need to be approved by the landlord as well. Until then, you'll of course need to full-fill the lease obligations by yourself. If it reasonably takes you three months to bring in the new roommate then that is your damages AND perhaps the half security deposit if you have a writing to that effect. You'll file a small claims action for that amount. Hope this helps.