I totally understand how you feel! I think when someone offers you the care, affection, intimacy or empathy that you have severely lacked in your life it is easy to feel swamped by the "good" feelings it brings. When you isolate yourself for protection, when someone gets behind your barriers i think the feelings are akin to realising your starving and need to fill up quickly. The good feeling becomes like a drug and encompasses you. It's hard to balance out and not feel so desperate for it, as if empathy and care are in short supply and it will run out any moment, it's like a cycle of feast and famine. It's hard to trust that your T will be able to cope with the power of your neediness and since you don't want to lose that, you become scared of your own vulnerability and you begin to withdraw from that source of caring in order to protect yourself from the (perceived) "inevitable" famine when the source of caring rejects you or withdraws from you.

That's how it feels for me at least. I hope that made sense :/