If she has refused to work with you, she has violated the contract and you can work with another agent. When one side violates a contract by not fulfiling its obligations, that breach allows the other side to treat the contract as terminated. If she or her broker try to sue you for a commission later, you'd have to be able to prove in court that they are refusing to do their job and so are breaching their obligations. Written documentation is invaluable in this regard: ideally, something in writing (even a text message) from her that she won't work with you or, failing that, at least a written record of a number of messages or contacts (e.g. emails and texts) sent by you to them, documenting their lack of effort and asking them to help you find a house--if you can show that you repeatedly asked them do their job but they nonetheless did not, that goes a long way to establishing their breach.