Next time you register for a trackday, register for a lower class. Force yourself to drive at least 30 minutes without using ANY brakes. Let go of the gas and let the car roll. What you will see and what really destroys lines and therefore your laptimes is braking.
Remember your cornering speeds when entering the corners at the end of the half hour. You'll see that if you've driven the track 20 rounds without brakes that your cornering speeds will be at least 20% higher then what they are when you brake late and hard and then step on it when exiting the corner.
Next thing is to ride the same track and brake light. Just as you would do when you approach an intersection on the street. Rinse and repeat and start braking more heavily (and therefore later) with every round that you do on the circuit. Look at your lap times and be amazed. They will steadily progress and will be MUCH faster as you drive around at this time without begin exhausted.
Braking late and hard is what get's you exhausted. Almost everyone applies to much brakes before entering a corner. The incoming speed is usually way too low. But all this constant braking and accelarating and being 'at it' is what's getting you exhausted.
Please try it and let me see how it worked out for you. When I got these instructions at first I was looking at my instructor and asked him if he was mad. This technique also works excellent for bike racing on the track.