The functions are parts of our psyche, really. People often compare them to lenses. They're what we use when we look at the world: we look at it through different lenses. So because we don't want to blame ourselves, we blame our functions, distancing so much from them that we forget they are actually part of us.
Our perceiving functions are the "lenses" with which we take in information, how we see the world. Our judging functions are the lenses with which we judge that information.
Very theoretically, a function:
A form of psychic activity, or manifestation of libido, that remains the same in principle under varying conditions. (See also auxiliary function, differentiation, inferior function, primary functionand typology.)
Jung’s model of typology distinguishes four psychological functions: thinking, feeling, sensation and intuition.
"Sensation establishes what is actually present, thinking enables us to recognize its meaning, feeling tells us its value, and intuition points to possibilities as to whence it came and whither it is going in a given situation."["A Psychological Theory of Types," CW 6, par. 958.]
Though all the functions exist in every psyche, one function is invariably more consciously developed than the others, giving rise to a one-sidedness that often leads to neurosis.
The more [a man] identifies with one function, the more he invests it with libido, and the more he withdraws libido from the other functions. They can tolerate being deprived of libido for even quite long periods, but in the end they will react. Being drained of libido, they gradually sink below the threshold of consciousness, lose their associative connection with it, and finally lapse into the unconscious. This is a regressive development, a reversion to the infantile and finally to the archaic level. . . . [which] brings about a dissociation of the personality.["The Type Problem in Aesthetics," ibid., pars. 502f.]
Here's how you (if you're stuck between Ne and Fi) can go about discerning your dominant.
1) Understanding whether you prefer introverting or extraverting. Introverting means you're orientated to direct your energy inward: you prefer busying yourself with your thoughts, ideas and concepts. You like being busy in the inner environment and it's what cost you the least energy to do, gives you the most energy actually. On the other hand, extraverting means you prefer the outer environment: busying yourself with objects in reality. People, nature, whatsoever that's not inside your head. If you're an extravert and you're being busy with inner stuff, say you were Ne-dom trying to focus on Fi stuff, it would exhaust you more, cost you more energy than when you're leaning mostly on your Ne.
2) Identifying your inferior function. The inferior is interesting because it leads to pretty characteric behavior. I've linked sources in the first post, but I'll link it again below. It must be said though that sometimes you may experience some influence from the tertiary.
- INFP has Te as inferior and Si as tertiary.
- ENFP has Si as inferior and Te as tertiary.
3) You're prone to 'using' your dominant more than your auxiliary, because it's the function you prefer the most. As to that, only you can really tell which one is more dominant in your head. :)
So basically you find your dominant and inferior. In your case, finding your dominant will tell you automatically which is the auxiliary.
(Sidenote, xNFP's don't exist, because they don't have one dominant, whereas per definition, everybody has one dominant.)
Anyways, as for your question about Fi-suppression (that's what I'm coining it for now, I think that's what you mean, no?).
People's type or preferred cognitive function order doesn't change. Remember that they are about how you perceive and how you judge. What does change is your behavior. Sometimes you're under the influence of moods for example, which make you act different than you usually do. For instance, I know I'll fill in type questionnaires differently if I am in a mood, or have recently been emotionally affected by something. There are all kinds of things that can make you behave like you're "suppressing" Fi. But whether you're truly suppressing Fi remains to be seen per occassion.
You have all four functions 'available'; the other four fall in your shadow. 'Available' in the sense that your dominant function is conscious and the fourth/inferior very much unconscious. I don't know much about the shadow functions, other than that the inferior is like the gateway to your unconscious and the shadow functions are like the contents.
You lean mostly on Fi as an INFP. Sometimes you'll lean more on your aux. I've seen it happen as something that I call Ne-overdrive: being obsessed with possibilities and not being able to come to a conclusion - which implies not leaning enough on your Fi and Te to reach conclusions.
There's also the Fi-Si loop, a loop in which Fi fuels Si and vice versa. I guess in your experience, Fi should still be pronouncedly present but accompanied by a strong experience of Si.
Now the inferior is actually quite... "notorious" for compensating one-sided leaning/usage of Fi (and therefore helps with typing yourself correctly). It usually arises when you're stressed out and is often identified as a short term thing: having a sensitivity associated with this cognition, having outbursts. But there's also a long term thing which is called "being in the grip". It's all pretty much explained here:
Recognizing the Inferior Function in IFPs (worth the long read!)
Being in the grip, or having an outburst, you could see that as 'suppression', I guess. However! It's behavior based on the inferior. It doesn't change your dominant function. In fact your dominant function doesn't change ever. But you can definitely act and behave sometimes like it wasn't your dominant function.
See what I mean? So yeah, Fi-suppression is sort of possible, depending how you'd define it, but you won't lose your dominant function.