The first thing I did when I installed the game was turn off all assists, which meant I spent the first few hours spinning out on every corner. But it was ultimately worthwhile because I was forced to work it out. It eventually clicked for me when I realized this was a rear wheel drive car, and therefore engine force is *always* along the axis of the car no matter how the front wheels are turned. At that point I stopped hitting the throttle too early on corners and life was good. I also learned the meaning of the word understeer.
I suggest learning unassisted control using time trial mode, because that gives you optimum grip, hence is most forgiving, on a relatively simple circuit; Montreal or Bahrain, Istanbul or Hockenheim are all reasonable choices. Start really slowly, all assists off, and if you get round the track without coming off then that's a victory, even if it takes you five minutes. Then on the next lap, go a little bit faster. Get a tiny improvement on each lap, and it won't be long before you've got some respectable times.
I confess I disagree with the idea of going straight to AI legend mode. It's just very depressing, and in any case the AI is grossly unrealistic. Racing against ghosts from time trial will give you a better feel for how a real driver would tackle it. Pick a ghost that did its time with no assists (the tyre track symbol to the right of the name will be turned off), pick the same car as you're driving, and pick one faster than you, but not so fast that it gets away and you can't see what it's doing.