A lot depends on what you are discussing before you go into it. Perhaps what happens is if a door is opened where there is too much crammed in that place behind that door, it can be something that is so overwhelming that one simply doesn't even know where to begin to talk about it, so disassociation takes place. It can even be that a therapist is reminding you of someone that you have struggled with in your past where you just shut down because you are not comfortable, but might not be aware of it.

I disassociated and avoided talking about my older sister, and anytime anyone I interacted with was like her or reminded me of her I was triggered. Well, I have been talking about her with my therapist finally because of how she is challenging me and triggering me badly. With the behavior patterns I have explained that she presents to me, as well as others and her obcessive need for control, my therapist has told me I am descibing the behavior patterns of someone who has borderline personality disorder. It is no wonder I had such a hard time talking about her, how my mind gets very confused about her because she has a lot of toxic behavior patterns that confused me.

So, while it's hard, even just a little piece at a time can eventually get you to a point where you can slowly start talking more about whatever is behind a door that whenever opened becomes so very hard to talk about.