It may be the car setup that's giving you the problem, but with an oversteering car, you should be ok unless it's too extreme.
The first problem corner, slow down enough but not too much and start getting your car on the apex near the entrance. The corner is angled so your car will most likely want to fishtail if you get it on the right point. Just countersteer through. You chould be able to go from the outside to the inside and out again with no problem. If you hit the outside at the exit, slow down more. If you hit the middle inside, increase your speed.
Problem Corner 2. This is another angled turn. I had the same problem before, mainly with scrubbing off enough speed to drift properly. GT4 will tell you to brake too late. Start braking earlier so that you can get rid of your momentum from the long straight. Start the drift later with a smaller angle. At this low speed, you can increase your angle as you go along. I'm still working on going in and out faster but it may not be possible.
Problem Corner 3. This corner is exactly the same as the corner before in terms of angle. just make sure you slow down enough and start your drift later and you should be ok.