Oh gee, turn the pressure nonsense off.
If you insist on having it turned on, you'll need to go to the Brush panel (accessed via the Window menu) and play with the spacing, the smoothing, and (I think) the shape dynamics.
But seriously, turn it off.
Well, I have to think about the method I suggest to you, you see? What works for this photo might not work for another photo.
So, could you show me one or two other close-ups of the problem as well?
This should work:
Download PSD file
Add the Channel Mixer layer first. Then go back and make the Levels layer below it, invert its mask, and paint on gently until the darkness is gone.