Don't be discouraged. Do you know how the functions form at type?
There are four judgment functions (Fi, Fe, Ti and Te) and four perception functions (Ni,Ne,Si and Se). A type is formed at its most simple description, by it's top two functions(a judgment coupled with a perception function) - that is, the functions that an individual feels more natural using. With this, we have 16 different pairings, the 16 types. We call a type P or J, in the simplest sense, by looking which of that top two functions is extroverted.
For instance, INTPs are Ti-Ne. As Ne is a perception function, INTP is a perceiving type. INFJs are Ni-Fe, as Fe is a judgment function, they have their J. So, when we look at a given type, we can already know which functions are they using by looking at their four letter code.
But it doesn't stay at the top two functions, as you may already know. When we have a function, we have an opposing function to that same one working. It has a different preference (N vs S, T vs F) and attitude (Introverted vs extroverted). So, when we have Ne, we have Si as well. When we have Ti, we have Fe as well. The more we use a function, the weaker will be their opposite's interference.
Thus, we'll have, for the INTP Ti-Ne-Si-Fe. Those four functions are the conscious side of an INTP. He/She will have a preference to use these functions regarding each one of the functions preferences. And you'll notice that they're into a specific order, the order of use.
The most used function is the dominant (or hero) function. The second most used function is the auxiliary(Or parent) function. Then we have tertiary (Or child/puer) function and inferior (or anima) function.
But a given person is able to use all eight functions throughout their lives. It may not use it skillfully or consciously as their top four functions, but they still use it. The other four functions are the shadow functions, which are unconsciously used, mainly. The ordering of the shadow functions are the same as the conscious functions, but their attitudes are switched. And also, for the shadow functions, each of the conscious functions role is distorted.
For the hero, we have the opposing personality (Te, for INTPs). To the parent, we have the senile(senex - Ni, for INTPs). To the child, we have a trickster (Se, for the INTP) and for the anima, we have a demon (Fi, for INTPs). When you read that article about these archetypes, you'll notice how they play a role in our fears and desires and relationships.
So, in the end, we can list all eight functions of a type. And that's also why it's not really plausible for a person to change a type. If you changed types, you would have to change your psyche configuration heavily. It's not about the change of one function - or letter, it's about changing your drives, fears, attitudes towards people and things.
I hope that this information is helpful. If it is, it may help you to read that article more thoroughly. I also recommend reading some of Adymus' posts on MBTI and typology subforums.
When you start learning in depth about the functions, the theory gets a bit clearer. It's not a definite tool, but it's rather good to understand how people - and yourself- work.
Edit: At the part I said 'weaker the interference' I meant that it's less comfortable to use in opposition to the other function.