Learn the car better, there is no other way around it, a slow driver is cause of his lack of confidence in the potential of his car either cause A) he does not have a firm grasp of how far a cars can be pushed in general or B) Doesn't have a firm grasp on how a particular car can be pushed. Assuming you are following standard racing line and practices for a a quick time..its just a matter of learning the limits of the car.