The jab is a punch that is the product of not being tense. You must be loose, as in your muscles should be not tight, and the technique of the punch is very important. Exercises such as shadow boxing with hand weights should be under taken with care- for example do not snap out straight punches with hand weights as it is deleterious to your shoulders.
Try "catching the fly" with your left hand. Shoot it out lke you are trying to snatch $ out of somebody's hand. Learn to time the movemment of your left hand with the movement of your left foot.