It sounds like truth. It has the tone of authority. It arrives in the first person, which makes it feel like your own honest assessment of yourself.
It is not your conscience. Your conscience tells you when you have done something genuinely wrong. The inner critic tells you that you are wrong, not the action, but you, the person.
That voice was not born in you. It was learned. It is built from criticism absorbed when you were young enough to believe it, from standards that were never realistic, from environments that required performance to earn acceptance.
The content of the inner critic is not fact. It is a recording. And it plays loudest at exactly the moments when you most need to hear something else.
You do not have to believe everything you think. Especially the thoughts that make you smaller.