Hi Jordy,
I can definitely relate to not wanting to share certain things with a therapist or doctor... I have always thought I was pretty candid until this last year until it came to admitting to myself and to others how immature my attachments were. I have literally sat in my NP's office trying to say it, and then saying "I can't" after a moment. Trying again, and "I can't." Because I am afraid of the reaction...
Don't beat yourself up for not talking about it yet, but I believe this is an important thing to discuss with your T because it is a core problem for BPD. Its time to address it, you are in pain and the solution you have is not realistic, there is actually a far simpler one, hopefully.