Past experience probably means abuse/manipulation. People in abusive relationships sometimes have to learn to hide any feelings that could be used to manipulate them. I know some narcissists personally, and I am very careful to give them no more information than they need. They use the VERY few things that they have to mess with my feelings anyway. I never let them see any sign that anything bothers me, because that will only give them fuel.
I used to put all my thoughts and feelings on display, but now I am more careful about who I trust with them. I haven’t been abused or anything, so for me this is more of a conscious choice. For people who have been abused, it can be more deep-rooted, like a survival mechanism or a way to feel safe. I’ve been around people who have been abused. Earning their trust takes time. Took me like a year. Different for everyone. For me it was a year of consistently showing love and respect.