Why do I struggle to open up?
You struggle to open up because some part of you learned, at a specific point and for a specific reason, that being known was more dangerous than being alone. That is not a character flaw. It is a conclusion your nervous system drew from evidence, and it has been running quietly ever since.
What is actually happening
Somewhere back in your history, telling the truth about what you felt or needed produced a bad outcome. Maybe a parent got flooded and made your feelings their crisis to manage. Maybe you said something true to a friend and it got repeated. Maybe you were the steady one in a chaotic house and learned that needing anything made you a liability rather than a person. Your brain filed that outcome under a simple rule: disclosure equals risk. Every time you feel the urge to say the real thing now, that old rule fires first, usually as a tightness in the chest or a sudden urge to change the subject, before you have consciously decided anything.
Why this makes complete sense
A rule that gets reinforced even a handful of times, especially in childhood when you have no other data to compare it against, becomes load bearing. It is not irrational, it was accurate for the environment that taught it to you. The problem is not that you learned caution, the problem is that the rule never got an update once you left that environment. You are applying a policy written for one specific person or one specific house to every relationship you enter now, including the ones that would actually hold you fine. That mismatch, not weakness, is the whole mechanism.
What actually helps
Stop trying to force a big reveal and instead test the rule in small, reversible doses. Tell one person one true, low-stakes thing, something like admitting you are tired instead of saying you are fine, and watch what they do with it, not what you fear they will do. Your nervous system updates on evidence, not on insight, so a single calm response from a real person does more than another week of understanding the pattern intellectually. Notice, too, who you already do open up to without thinking about it. That person is data. Whatever made them safe is the exact ingredient missing everywhere else, and naming it gives you something concrete to look for instead of a vague hope that trust will just arrive.
The cost of staying closed
The guardedness that once protected you also quietly rations how known you get to be by anyone, including people who have already proven trustworthy. Left unexamined, it does not stay neutral, it slowly convinces the people closest to you that you do not need them, which is a different problem than the one you started with. The goal is not to become someone who tells everyone everything. It is to notice when the old rule is firing in a room where the original danger is not actually present.
When this runs your life, it usually traces to one underlying pattern. For this, it is most often the Keeper rhythm, the thing under the behavior.