Sorry to hear about your experience! It sounds as though your Realtor is not listening to you and your fiancee and I know how frustrating that can be.
My first response (as a Realtor) would be to tell you that you need to have a talk with your agent and discuss your concerns about her service, giving her a chance to remedy what she is doing wrong. Many times there is simply miscommunication between client/agent which can be easily fixed.
If, however, you feel that you definitely want to change Realtors here is my two cents.
Have you signed a buyer-agent agreement? If so, you need to check the fine print of the contract and see what it would take for you to cancel the agreement and find another Realtor. If you haven't signed any sort of employment agreement I would inform your Realtor that you have decided to find a Realtor who you think will do a better job for you, and leave it at that.
I actually don't elect to use buyer/agent agreements either but I am a bit more refined when I explain why I don't.
I tell them, "If you aren't happy with my level of service at anytime I feel you should have the choice to walk away with no obligations," and as long as I keep my word things always seem to work out!
Best of luck finding a new Realtor!