I've read that therapy for personality disorders is not always covered by insurance, so that might be another reason why your therapist gave you a different diagnosis, if you're paying through insurance. Plus maybe therapists don't want to put something in your medical records that could be a disadvantage down the line. Right now [eta: in the US*] insurance can't deny you for a pre-existing condition, but in a few years who knows. Being officially diagnosed with a personality disorder could cause problems.

I'd talk to the therapist about what the psychiatrist said, see what the therapist thinks about it and talk about her reasoning for the diagnosis she gave. Also maybe think about whether you feel you've made progress in therapy or not, because in the end that's the most important question--has it been working? Could it work better? Maybe the new medication will help change things up, but if you're on the right track don't consider that three years wasted.

*sorry, just saw you're in Australia and realized this may not apply