I was the same way for a long time. I could barely share anything for the first couple of years of therapy. My therapist thought that I experienced what some people refer to as "toxic shame".

I still haven't read Brene Brown's books, but I really relate to the articles and interviews with and by her that talk about shame and vulnerability. I had another therapist visit me at home recently and my shame reaction went into overdrive at the mere possibility of her in my house. She reminded me of Brene's work and said that it's a strength to show vulnerability (and that idea does help me to risk being slightly braver with how much I share).

I also find that it only lessens if it is acknowledged and confronted. Often initially I had to write stuff down for my therapist. I think it's something that takes a lot of time and practice.