The hardest races for me in the A class was. A5, 7, 8, and 9.
Im using the Sixaxix controller, and im new to using these kinda controlers, as i was alwys a PC gamer, so i had also a hard time just to getting used to the sixaxix.
Anyway
A5, A7, and A9 well its mostly about knowing the track since it is the same track, once you driven it lots of times you start to get the hang of it, and eventualy you will be able to at least get bronze, wich will let you pass the trial, you can always try more if you absolutely want gold.
A8 ( Ferrary 1 lap daytona road course challenge )was also about to drive me nuts, i had a hard time to understand how people would actually use this event as a cash cow, as the best i could do was position 6 or so. Until i finally just tried to drive it really dirty, you can basicly just use the other cars in as you brake, sadly its much more efective than trying to steer round them, as they always seem to bumb into you anyway.
I used the Ferrari F430 in the open car races, as this is the car i seem to be able to handle best

And in the Under 300 Japan class i used the NSX - R
Also if you have problems with a particular race, it can help to try and go into time trials, and see how the fastest guy with the car you use on the track you are drivin was driving. see wich gear he was in at the various turns where did he brake ETC...
And finnaly also try search for videos at you tube can help.
Anyway, im sure you will do it soon, as afterall they are not that dificult.