Why can I not be myself around other people?
You cannot be yourself around other people because somewhere earlier in your life, being fully yourself did not get met with safety, so a different version of you learned to go first. That version is faster than your conscious choices. By the time you notice you are performing, the performance has already started.
The switch happens before you decide anything
What you are calling fake or stiff is actually a prediction system running ahead of you. Your nervous system scans a room, a tone of voice, a pause after you speak, and it makes a call about what is safe to show before your thinking brain weighs in. If you grew up with a parent whose mood changed the temperature of the house, or a friend group where the wrong opinion got you laughed at once and you never forgot it, that scanning became permanent equipment. You are not choosing to hide. You are running a program that was built to keep you out of trouble, and it fires in rooms where there is no actual trouble anymore.
It made complete sense when it started
This is not a character flaw or a sign that you lack a solid self underneath. A kid who adjusts to an unpredictable parent, a teenager who learns which jokes cost him friends, an employee who watches which opinions get someone frozen out in a meeting, all of them are doing something smart. Masking is a survival skill with a good track record, which is exactly why it does not turn off just because the danger did. The problem is not that the skill exists. The problem is that it cannot tell the difference between the boardroom that fired someone for disagreeing and the dinner with people who would actually be fine with the real you.
Notice where it turns on and off
There are almost certainly people or places where this does not happen, an old friend, a sibling, one specific coworker, a dog. That gap is data. It tells you the block is not about your personality, it is about specific signals your body has learned to read as risk, like a certain kind of silence, a certain kind of eye contact, a raised eyebrow that once meant something bad was coming. Start tracking exactly what precedes the shutdown instead of just noticing that it happened. Was it a question that felt like a test. Was it someone with more status in the room. The pattern is usually narrower and more specific than 'I can't be myself around people,' and narrow patterns are the ones you can actually work with.
The fix is not confidence, it's evidence
Telling yourself to just relax and be authentic does not work because you are not dealing with a mindset problem, you are dealing with a trained reflex, and reflexes update through repeated evidence, not pep talks. Pick one low-stakes room, maybe a checkout line or a group text, and let one slightly more real reaction show, a real opinion instead of the safe one, a joke that might not land. Your body needs to survive the moment it was afraid of, on purpose, at a scale it can handle, before it will recalibrate. Do this enough times in enough rooms and the scanning system slowly reclassifies those rooms as safe. It is slow and it is boring and it is also the only thing that actually works.
When this runs your life, it usually traces to one underlying pattern. For this, it is most often the Mirror rhythm, the thing under the behavior.