You just have to get good again, simple as that. Pick a few cars that will allow you to cover a majority of races, tune them to where they handle comfortably for your driving style (trying to strike a balance between speed, acceleration, handling, and stability), and start learning the tracks in those cars.
Do time trials and see where you're struggling or where you can make improvements. Turn on track limit penalties and get an idea of how much of the track is actually available to use; push harder and harder into turns or cut corners more and more until you start getting track penalties. Adjust your tunes to be able to hit those limits. Watch other peoples' time trials and observe their acceleration, braking, and when/where they start turning into turns you have trouble with.
