I rarely feel better after a therapy session. I feel raw after a therapy session, like my nerve endings are ablaze and the slightest touch will hurt so much I won't be able to stand it. I'm fortunate that I'm able to have as much alone time as I need after session to regroup but it sounds like you might not have that luxury? Could you talk to your therapist about techniques you can use after session to ground yourself, to feel more in control of your emotions, to feel more connected to reality?

One thing about having BPD is that we have to learn to tolerate negative emotions. We were supposed to learn this as children but no one ever taught us. I've found that mindfulness is helpful in learning this skill but there are other ways to do it as well. As you learn that negative emotions aren't going to swallow you whole, they aren't going to cause you to go crazy, they just feel like it... it becomes easier to bear them.

It will become easier to talk to your therapist about what you're feeling without getting overwhelmed but right now you're just completely overwhelmed by your feelings, is what it sounds like to me. It took me months with my most recent (current) therapist before I could hear some of what she was saying because I was so deep into my own feelings. It takes time. Be patient with her and with yourself.