try the tutorials. I seem to remember GT having excessive tutorials.
Anyhow, have you tried coming in from opposite side and cornering?

I also recall that most levels have an area that you can go off of the tar itself onto a runway of sorts. For help cornering, try lifting the car as high as possible.
also check gamefaqs.

@theprof00

I don't think GT5: Prologue has a tutorial mode to show off for newcomers. I didn't check I just turned my PS3 off. I did a lot of cornering and managed to get 3 place on this last race I did. I'm doing pretty good so far. I was just a little frustrated earlier because of the learning curve its required. Like you have to memorize the track well. So I have to do a minimum of 10 times of the same track to learn the patterns.

There are runway parts and I use those a lot. I try to go in the grass a little not to much but on corners. And I don't think you can tune the car to your liking until you unlock the S Class.

I tried Gamefaqs and the walkthrough is just plain. It was only one faq available.