The suggested gear is always very conservative, you can easily do any curve with a superior gear than the one showed.
Honestly I don't know what so difficult about the manual, just remember to wait for the odometer to flash before going up with gears, and when down-shifting, it's needed to be done gradually while braking, being sure that the inferior gear will land at good regime. This way the car should slow down faster and also should be more stable.
Also, you'll be more proficient if you learn to count the gears (for example you are in 5th and need to shift in 2nd gear for a slow corner, just count down 3 times) instead of watching the indicator on screen, this can help you to remain focused on what's happening on the track.
With all of that said, and sorry if I said something stupid, I'm not near decency regarding lap times nor being competitive in most of the races, I just think to know how to use a manual shift. And I really have a lot of fun with it, I think it's like half of the driving.
Also, I have a question about the dynamics during a corner. I noticed that in most cars, re-accelerating gradually mid corner helps the car to regain grip and fight the understeer, and it's perfectly logic if this happens with RWD cars, because by accelerating the weight will go to the rear wheels, regaining traction. But this seems to benefit the FWD cars too. If I accelerate and lift the weight to the rear, shouldn't the car have even less traction? I don't get it, is this a limit of the game engine, or this works in real life too? Take into account that when this happens to me, I think that the wheels aren't blocked because of the ABS on and the steer still works to a certain extent.