You might just need to go back to the drawing board gk. This series isn't hard as long as your racing fundamentals are sound. Probably all this time you've gotten by in GT by using power to compensate for an underdeveloped racing technique. It would benefit you to improve your fundamentals because if you can't beat the Vitz series, you're going to have a hell of a time with the F1 series.
Run some time trials against your ghost for a while. Find faster ways through the corners and learn how to preserve momentum. Familiarize yourself with the braking points on the tracks. This is something all of the faster drivers have done. I myself have literally sat down and run a hundred laps on some tracks, learning nuances, bumps slippery spots and how to manage car attitude and weight transfer.
Take the tests over, go for gold or at least silver. Don't settle for just passing because your goal is to get better, not to complete them. Really get into the mechanics and physical aspect of the cars you drive, their differences, weaknesses and strengths. Take the time and make the effort to reach that next level... where you really feel in control at high speeds.
If you do it, I swear, your level of enjoyment while playing this game will greatly increase. The ACT OF DRIVING will become fun and the game no longer is simply something to pass time and work on completing. Go ahead man, dig in.
If you really want to improve fast, you could race against some other people's ghosts. If you have a SharkPort, I'll make some Vitz ghosts and send them to you for practice racing against.