I wouldn't diagnose yourself, but if your therapist is good then you should be able to say to your therapist. "When I was getting ready for this appointment I was doing research about my symptoms and found that a lot of them are the symptoms for borderline personality disorder and I find it confusing. Can you help me understand this information?"

Or something like that. And your therapist should be able to give you a reason why you were diagnosed with bipolar disorder. There's always a chance your initial diagnosis was wrong or made with incomplete information.