On the license tests in the NA/NTSC version, you can't adjust the driver aids, unfortunately.
But for your own racing, I'll repeat the advice that you turn both the stability controls (oversteer and understeer) ALL THE WAY OFF. In a race with my new Integra Type R, where I had forgotten to turn them off, the so-called "stability control" actually threw me off the track at Sears Point (Infineon), a course I know extremely well, during a corner where the car was doing exactly what I wanted it to do. At the top of the hill I was counting on the suspension unloading to bring the rear end around, but as soon as it did so, the stability control locked up the outside front wheel and yanked the car right off the outside of the turn. So the "driver aids" actually hurt more than they help by interfering with your understanding of how the car is responding to your actual control inputs.
kestrel, based on your first post in the thread, it's clear that you are simply overdriving the car. Turbo and nitrous both mean you are counting on straight line speed to dominate the other cars. This is called "point and squirt" and, while it may be effective sometimes, it will never teach you how to drive.
As everyone else has said, practice patience and repeat the SIFO mantra to yourself.
As you approach a corner, you'll note that a number appears on your screen near the guages. This is the suggested gear for the upcoming turn. It begins to flash at the recommended braking point. It's not perfect, but it's a good guide for you to get your bearings with. Start braking when the gear indicator number blinks, and downshift steadily to the recommended gear. Get the car slowed down before the turn, then release the brakes and turn in. Once the car is settled into the turn, get back into the gas, but don't jam it down. The trick is to avoid overdriving the car by trying to keep your control inputs smooth and in sequence:
BRAKE >> TURN >> THROTTLE >> STRAIGHT
GT4 really punishes heavy control inputs, as it should to be realistic. Your best bet is to get a wheel and someplace sturdy and comfortable to mount it. At the very least, make sure you use the analog sticks for both steering and throttle/brake. I was a long-time GT3 player, 100% complete and all golds using the Dual Shock 2 (DS2) controller that way. However, GT4 really requires smooth and gentle driving, and it was frustrating with the controller. Since buying the wheel I've been getting better and better and even though I don't get to play very frequently I've got all licenses silver, and more than half of those gold, including some of the upper-level licenses. I'm not a super-human player and I'm low in the online racing (OLR) ranks, so if I can do it, you can do it too with practice.