*hugs*

Getting diagnosed can be a confusing process. Like Onward pointed out, many illnesses have overlap and similarities.

Diagnostics are meant to be strict, objective categories with clear boundaries (because the dominant approach to mental health treats mental disorders the same way as it does physical illness)--but we're people, and these are our minds, not our bodies. We don't work that way! We often fit into more than one box, and mental illness is a complex, fluid, and confusing thing... Avoidant has things in common with Borderline, and Borderline has things in common with Bipolar, and there's a lot of other intricacies and connections in there, too, I'm sure... Being confused when trying to figure out what's going on with yourself is a completely normal thing.

If it's any help, my situation is similar to yours. I was recently diagnosed with both bipolar disorder and avoidant personality disorder. Both disorders can be diagnosed simultaneously, since the former is a mood disorder, while the latter is a personality disorder. But also like you, I used to think I was borderline, not avoidant! And while I'm coming around to the idea more (I had an epiphany the other day, realizing that the Avoidant diagnosis explains so much of who I am, what I do, and how I interact with people), I still feel there are certain traits that fit more into the borderline box. Since I don't believe in clearly defined boundaries between mental disorders, I probably have elements of both. Perhaps that merged version which Onward suggested. :P

It sounds like you have some time to think about it, between now and your next T appointment. Bring your thoughts to her, when you next see her. If you still don't agree with the diagnoses after speaking with her, you can seek a second opinion. Different professionals will often diagnoses different things (objective science my foot ;P), and you are an active participant in your therapy. You know what you experience and feel better than anyone, so don't let others put a label on you, if you don't feel that label fits with your experiences.

And if it takes time to figure out what you feel best fits your experiences, that's ok too. It IS a confusing process.

*hugs*
Wishing you all the best! Please keep us posted on how things are going for you.