If you can drive reasonably well with the driving line on, maybe do some laps with it on, and then watch the replay from your driving view. I'm guessing the driving line won't show up in the replay anyway. Bring up the info bits so you can see where you're hitting the brakes, speed, what gear you're taking the corner, etc., as well as watching for where you're braking, turning in, apex, etc. As FuzzyFez said, there's a lot of stuff around on the internet, and really, most corners are covered by just a few simple concepts. Simple to grasp anyway... then you have to learn to actually do it on track.
Back when I first got into racing games I'd use the driving line thinking I'd eventually just be able to turn it off. But IMO, as long as you have it on, it's not really helping you move forward. Instead of getting used to sighting reference markers and looking to the apex and whatnot, you just stare at the line for all your cues.
As you get better at it, your braking points will get closer to the corner, you'll carrying more speed on entry and trailing the brake to the apex, and then getting on the throttle and smoothly powering out to the corner exit. You'll also get better at judging the specific situation you happen to be in at the time and adjusting on the fly. Having the perfect driving line all planned out and practised is great for a time trial, but in a race you have to be able to adjust if you plan on doing any overtaking.
I'll also say it's maybe not as big a task as you might be thinking. While you do have to learn your way around any new track (and of course the car plays a part too), once you have a grasp of how to do it, it'll come fairly naturally to apply it elsewhere. Aside from the fact that you'll have become better at it, if you break a circuit down corner by corner, you'll find a lot of what you're looking at are corners you've already learned to take navigate at other circuits.