2 things happen in a low-grip environment. Understeer becomes incredibly pronounced, and oversteer becomes incredibly pronounced. That is to say, the car will rotate faster, and will also refuse to turn more.
Using a mouse and, I presume, buttons for throttle and braking is going to be tough. If you had a wheel, I would suggest entering the dirt corners slowly, let the understeer build up, take an incredibly late apex and gently add power as you exit. Get used to balancing the car with the throttle and steering simultaneously. Once you get good at that, you can start using the brake to rotate the car into the same angle on corner entry, allowing higher cornering speeds. Using a mouse, I really don't know what practical exercises are going to get around the permanent roughness of the inputs.