Have you ever given up on any certain exercise, sport, or competition? Believe it or not, your body can go WAY further than your mind believes it can. Your mind gets used to whatever your normal level of activity is. The moment you try to push past that, a huge alarm goes off in your mind telling you, "Wait! This is hard! Slow down! This isn't possible! You are dying!" I'm sure you have been there at least once. I'm here to tell you that if this is happening within the context of safe and controlled exercise, your mind is probably lying to you. The only way we get better if we continually press the limits of our physical ability. That is the only way we improve and better our physical physique and performance.
So, the next time you feel like quitting in a workout because it is getting harder, or you are breathing really hard, or because your muscles are burning, try to embrace it a little longer. Push yourself and you may shock yourself at what you are actually capable of. Then your mind will reach new heights of belief in your ability.