I doubt it's you who sucks at GT4, but rather, GT4 that sucks FOR you. Yeah.
Every new GT game, they keep changing a lot of the physics and gameplay controls a little bit, so it becomes harder and harder... and makes you go out and buy their stupid racing wheel.
I remember in GT2, I did every damn race with my good ol'd D-pad and X and Square buttons. But now trying to do that in GT4 is so much harder, especially when they have the analog buttons turned on at all times, the pressure sensitive shit... you always have to press so hard on them to go sometimes.
So if you really want to perform best, get the wheel...