Martial artists must learn to differentiate between extreme discomfort, and pain. You cannot get tough if you are unable to train, so you must know when to stop so you may recover and be able to continue your training.
To get tough, martial artists need to expose themselves to stress so they may develop skills to deal with the stress. This is not as complicated as it may seem. All you need to do is stress your body by doing more than you did the last time. By lifting more weight, running more wind sprints, or doing more sit-ups or push-ups, etc. you may safely apply stress to your body. Doing what you can easily do does not cause stress, you must always try to do more.
Since people usually act the way they feel, you should learn to act tough. Feelings and emotions are expressed through the muscles of your face, shoulders, arms, and legs and in your breathing. Martial artists must learn how to control their muscle movements and expressions to so they may display a confident, tough exterior, even when they are feeling beaten. You may do this by keeping an erect, confident posture and using acting to display the emotion that you want to portray. However, a fake emotion may quickly turn into a real one, so be careful.
To be tough you must be recovered from previous fights and be able to recover quickly from injuries that may occur in the current fight. Always allow adequate time for injuries to heal, before fighting again. When training and before a fight, get plenty of rest, eat properly, and avoid alcohol, drugs, smoking, etc. Before a fight, eat a diet that will stabilize your blood sugar so it does not bottom out during the action.