It hurts so much because as you move closer to your T, feelings that you have pushed away and denied are surfacing, as your T holds out the possibility of providing for unmet needs. Those needs are from childhood and when they are not met, they can cause a tremendous amount of unacknowledged pain since we push the pain away to keep our parents "good" so we stay near them.
I wrote a long post about these feelings and why we have them on my blog, so if you don't mind, I'm just going to put a link here. I hope this helps.
Disorganized Attachment or Why You Think You're Crazy but You're Not
Bottom line, the most important thing is to keep going back to your therapist and talking about the feelings coming up, no matter how many times you need to do that. I won't lie, it's really painful to face these feelings, and can be terrifying beyond belief. But it's how you heal and can learn to have close, intimate relationships and move closer to people without all of this driving you away each time you allow yourself to get close.
And I do not think you are being selfish at all. I also do not want to pathologize your reaction by labeling it "borderline." The truth is that people who did not get what they need as children, and even worse, were actively abused, will have these kinds of reactions. In other words, you're not broken or sick, you're wounded. These feelings are a perfectly reasonable reaction to unreasonable circumstances.
But if we grew up in an environment where our normal, healthy needs were treated as an imposition or acting on those needs got us injured, then we learned to be deeply ashamed of our needs in an attempt to NOT express them and keep ourselves safe. We grow up, but we keep the shame. So when you see these feelings in the correct context, they make a lot more sense which in turn makes it easier for you to have more compassion for yourself. Which is what you clearly deserve.