I can't tell you specifically, but there are some things you can try. Do them one at a time and see what works for you.
Add rear toe in, not too much all at once though.
Raise the rear ride height.
Soften the rear springs.
Soften the rear dampers.
Add rear aero, if you can.
Play with your rear camber, although which direction will depend. Experiment a little.
Move your brake balance forward.
Failing that you may just have to learn to be more gentle with the throttle. It's a very, very powerful car. I don't have FM5, but in iRacing you have to be unbelievably careful with the throttle. You can only really floor it when the car is perfectly straight or you'll be fighting it the whole way. I doubt FM5 is much different.