Not sure if you are using wheel or DS3. I am sure you are already on race soft and already on maximum PP allowed.
If the problem is oversteer on turning, set SRF on, active steering to mild, rear toe in, set aero on the rear to the maximum your car can give.
If the oversteer is on braking, set braking bias 5:3 or even lower on rear. Ensure that heavy braking is done on fairly straight steering.
If the oversteer is on acceleration, set TC to 1 or 2, set the rear suspension with some toe in and be fairly gentle on acceleration.
Active stability management and TC above 2 usually slows you down so turn that off.
As mentioned in other threads, a Mazda Furai or R35 TC are well mannered cars without tuning. If you want straightline speed, go for Mclaren F1 GTR or Chapparal 2J.
Ignore complaints about how easy the seasonals are. This community has some outstanding drivers who rank in the leaderboard of time trials. They can drive their Kubelwagon and win the seasonals. For people like myself, I struggle with some of the seasonals too. In time trials, I am rank like 20000+th position (2500th if I try harder like in GT academy) most times so I probably belongs to the norm.
If you still can't win it, that's fine. If you persist, you may find victory all the more sweeter. Who knows, you might even be alien material. (I know I am not)