I have a new gp who makes incredible assumptions, and questions everything I tell her. Should I look for another gp?

If not comfortable . A patient should be with a physician with whom he/she is comfortable. It is a vital part of the patient-doctor relationship.
May be not. Taking a good history from a patient is very important, sometimes better than doing tests. If the intention is to understand your complaints better, then that is a good approach. She may be trying to help you understand. At the end of the day you have to do your own research and be responsible for your health. Doctors advice and guide you.