Well, you shouldn't be on the brakes hard AND trying to turn at the same time. Most vehicles will understeer in such a situation. Try to do your braking before the corner. Once you're at and/or close to the desired speed for the corner, turn in, moderate the gas until you reach the middle/exit of the turn. Hit the gas and slingshot out of the corner. That's obviously in laymens terms, but you should get the idea. Furthermore, if you are still noticing some understeer, you can try to stiffen the rear springs, or toe out the rear, etc...