I concur with the poster who said to take the cognitive functions test. Your uni may provide this test. However, I also know that the tests can be crap.
It's always been my opinion that the test shouldn't be taken multiple times. It's biased at this point because you can link what each question is asking to each letter. At this point, people start creating a checklist (Oh, I'm organized, therefore I'm J. I like going out to parties, therefore I'm E). Another poster brought in environmental factors, which I think is relevant, but complex to understanding what you are.
In any case, as for the functions
INFP = Fi - Ne - Si - Te
INFJ = Ni - Fe - Ti - Se
My experiences with Fi vs Fe (disclaimer: I could be absolutely wrong here and am basing this off of two people)
I dated an INFJ and an INFP on and off for a few years (err..not at the same time).
Ni - Fe strikes me as this:
The area around me is my world. I am connected to my world and the feelings and emotions within. These feelings and emotions affect me. Because I am connected to my world, I want to be proactive about the possibilities within this world. In a way, the Ni acts as a box for the Fe.
Fi - Ne strikes me as this:
I know my inner feelings and values. I want to expand these values to the outer world around me. In a way, the Fi is a box for the Ne. Fi is individual oriented and Ne is expanding possibilities. Thus, I want the people around me to explore the various opportunities of my value system.
It's always been hard for me to explain the differences between the two, though it's very obvious to me the differences, they are hard to verbalize. On one hand, the INFP was more in tune with his emotions, whilst the INFJ was more in tune with mine. The INFP was more emotional and reactionary, whilst the INFJ always needed lots of time to deal with his emotions and outbursts were rare (there were also plenty of warnings).
I can think of a few examples, but I'm not sure how helpful they would be.