You can be sincere and still be performing a version of sincerity you learned from others.

Sincerity has a style, and you learned yours. The pauses, the vocabulary, the way you signal that this bit is honest, all picked up from people you found convincing. That does not make the feeling underneath false. It does mean that even your most unguarded moments are partly a form you inherited.

There is nothing to fix here, only something to hold lightly the next time you feel certain that you are being completely real. And it is worth extending the same allowance to other people, whose sincerity you might be discounting because their learned form differs from yours.