Introversion and extroversion are not actually personality functions themselves; they're directional attitudes of functions S/N/T/F.
Basically, Jung says that each person has two sides to him:
1) A decisive and organizational Judging function T or F, and
2) A flexible, open and adaptable Perceiving function S or N.
Also, one of these two will be used to govern the outer world of interactions with other people and ideas and things, and the other will be pointed inward to govern the private inner monologue and your own ideas.
P types point the organizing drive inward; that is, they appear outwardly flexible and spontaneous because they keep the Judging process inward and more private.
J types point the organizing drive outward, preferring to show the world their decisive and scheduled side in most cases. The more flexible attitude is kept inwardly private and not often shown to the outer world.
If you want to be a balanced and healthy person, you need to learn to exercise both of these two processes in equal proportions. (For you as an INFJ this would be dominant Ni and secondary Fe.)
As an INFJ you place more emphasis on your Ni than your Fe, but what you should aim to do is to become xNFJ over time. Truly balanced people appear neither introverted nor extroverted in the long run because they have good command of both of these functions and know which works best in which situations.
Fe is in itself the ability to open up and use the emotions of others to respond properly and develop strong social rapport with people according to cultural and ethical community standards. The fact that this process is so slow and difficult for you indicates overdependence upon Ni and a comparative lack of development of Fe.