I'm not sure why so many people recommend watching the B-Spec driver to learn. Sure, he takes the correct line (more or less) but his approach to corners and braking is often very strange, such as hard initial braking, coasting into the corner, then additional trail braking into the turn...very strange tactics indeed, and perhaps explains some of the poor AI speed.
My advice would be to read a bit about proper racing technique, the racing line, etc. (if you don't already know this), then pick a well-behaved car under 250bhp (I recommend the Mazda RX-8 Type S, but I'm biased

) and a relatively straightforward track, perhaps Tsukuba or Midfield.
Drive around slowly at first, like REALLY slowly, like DRIVE it don't race it. Use moderate throttle and try shifting where you would on the road and just get a feel for the track at slow speeds, it looks quite a bit different actually. Try out the racing line at these speeds, get a feel for how the track flows. The best drivers don't drive corner to corner but rather maintain a momentum and flow throughout the whole track. As you learn more advanced techniques this becomes easier to grasp.
Once you've done a couple of Sunday drive laps

start to shift a little higher, push a little more, but maintain that line you practiced in your sighting laps. As you enter turns faster you'll feel the car behaving more on the limit and you'll get a feel for what's happening. Eventually you'll be pushing full throttle with more confidence. The key here is patience, patience to drive the track slowly and not give into the desire to floor it until you're ready 👍
And lastly, the time WILL come when you push too hard and the car gets out of shape. Remember this: if you feel like the car doesn't want to turn (understeer) through corners, try backing off next time around and take the turn slower and with more control. More often than not understeer is caused by overcooking a turn (unless we're talking about an FWD, then it's just terminal

)
Hope that helps 👍