That has been one of the biggest changes I've noticed in GT4 (from GT3). The braking sensitivty has been tuned up an amazing degree. WHile in GT3 a full application of brakes tended to give you the ability to stop, in GT4, it does nothing but lock the tires.
My suggestion is work on brake control. When you come in hot to a corner, only apply the brake 1/2 way. You'll notice more control, and less tire squeal. Less tire squeal = more traction.
From your bried and eloquently worded desription it sounds like you'd coming into a corner, and going full lock, full lock. As in tires all the way over, brakes all the way on. This equals an immediate loss of traction as you've reduce tire grip and tire direction.
Give it a try, where you use a slower car, and not as much pressure on the brakes and steering. Follow my advice after you've followed TankSpankers.
AO