The fixed expression ‘I must admit’ is used to express something that you feel guilty or embarrassed about, or to show that someone else impresses you more than you expected. For example:
I must admit, I’m not very good at this sort of thing.
I must admit, you’re quite a singer.
The fixed expression ‘you must admit’ is used to express concession, which means acknowledging a point made by someone else, even if you disagree with them. For example:
You must admit, he has a point.
You must admit, it was a close game.