What does Macbeth admit in this aside? Based on the lines what do you think is macbeth's tragic flaw ? Scene 4 lines 45-53

I think you are referring to the lines:

he prince of Cumberland! That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand, yet let that be
Which the eye fears, when it is done, to see.

This shows us Macbeth is angry that Malcolm stands in his way to be King. He displays his dark desires to be king any way he can including killing people.