I am Confused

Today, I told my boss that I won't be able to stay as a receptionist. I just have too much drive to just sit around at a job that doesn't challenge me in a way I am striving for. Assuming I would not be with them next week, I told her that my temp agency has found me a job that starts next week. And her jaw dropped, and was like no we haven't even offered her the job. Who knows if she will accept it. OK. So the day before she pretty much told me that she found another person, making me think that she will start next week. BLAH! My tenative last day of work would be July 3rd. I was so ready to move on but apparently they thought I would reconsider and stay with them forever. What can you do?