The sentence is some version of:

I am only safe if I stay quiet.

I am only accepted if I do not make waves.

I am only loved if I keep what I really think to myself.

The freeze is not a lack of courage. It is an old system doing exactly what it was designed to do. At some point — usually in childhood, usually in a situation where speaking up genuinely was not safe — the sentence was written. And it has been running ever since, firing in meetings, in relationships, in moments where your voice matters and your body says not now.

The standard advice is to practice assertiveness. Rehearse what you will say. Start small. That advice is fine as far as it goes, but it does not explain why your body overrides the rehearsal in the actual moment. The sentence does.

The work is not to push through the freeze. It is to find the sentence that created it. Once the sentence is audible — I am only safe if I stay quiet — the freeze has context. It becomes something you recognize rather than something that controls you.