Well it depends on whether you only lose it going into corners you brake for, or in all kinds of corners.

First of all, tighten the setup by moving weight forward so you can stay on the track in order to practice with it offline, and when you've mastered to drive it safely, you can loosen it back up a bit, learn to drive this as well and so on until you've gotten back to the 'original' setup that will be run in your league.
Posting a replay of your driving along with the actual setup would be useful as well, as there is a variety of possible reasons to lose the car. You might either turn the wheel too much, or you do not stabilize the car on entry with the throttle, and maybe the fixed setup's brake bias is also set too low for you, i.e. you don't use enough throttle to prevent the rear wheels from locking up under braking.