The biggest issues I had when it came to determining my type were a) deciding whether I was INFP or ENFP, and b) considering that I might be a thinker because I certainly seem like one and sometimes I even feel like one. There was also the issue of my understanding being clouded by stereotypes, bias and the behaviour I observed in others of particular types - that last factor had contributed to my confusion as to whether or not I was Ne-dom (despite the fact that I was very much aware of the fact that others and I had different methods for typing and defined functions or types differently), and I did briefly consider that I might actually be an SJ masquerading as an NP because NPs are often described as being so much cooler.
Also, another problem I had was in deciding on a particular typing method - every site describes the functions and how they manifest differently, and I was mentally conflating a bunch of different typing methods together to attempt to create my own set of rules by which to type, and it wasn't working.
I have noticed that some people do still type based on their behaviours (which I suppose works if you're not looking into functions) and even on the views and opinions they had (e.g. believing in destiny = Ni). I think I'd slipped into this mode of thinking every now and then, and I had to remind myself that it's not about what I do or what I think but rather about how I got to deciding to do what I wanted to do and how I came to the conclusions I did. It's tricky, but you've got to think about how you're thinking. I personally came to understand cognitive functions best when I was defining them on an abstract level (e.g. Fi = subjective value judgment, NOT staying true to yourself, despite this often being a manifestation of Fi).
Anyway, I'm an INFP because:
1. Everything gets put through a wringer. This is probably true of all introverts, but since I was having some trouble on the Ne vs Fi front, this was the deciding factor for me. I'm not manic in maximizing the potential of everything around me; instead I'll filter the possibilities I see and maybe want to give rise to through a straight line of judgment, and turn everything into a matter of value. This meant that a) Ne was being guided by a judging (and introverted) function, and b) I'm an F-dom.
2. I struggle to stop using Fi, but can go hours without using Ne. This one's tricky, because I think that one may be so used to constantly using their dominant function that it's difficult to acknowledge it unless there is something actively pushing against it. I'm also a little bit skeptical about looping; if looping is, in fact, a thing, then I highly doubt that one can loop for more than perhaps... a few days at a time. I feel stifled when I can't use Ne, but unlike Ne-doms, I can go without actively using Ne for extended periods of time, instead using Ne more as a tool for mental / intellectual stimulation than anything else. I think I had a point here, but I've gotten off track... I think my point was that there are times I can sit and bask in Fi and only Fi (and even Fi-Si loop for an hour or so), but I can't do the same with Ne despite the fact that I cannot repress Ne. Sorry if this didn't make much sense.
3. Jung's description of the Introverted Feeling type is on point. Okay, so his descriptions are hella exaggerated, and I can relate to the Ne one too. I ultimately settled on trusting his descriptions rather than the ones that seem a little closer to the Socionics (or the Myers-Briggs) descriptions because he's more abstract in his definitions. I had a lot of issues with most descriptions because many of them were just describing basic human behaviour (Ne = having ideas, Ti = analyzing etc.), and because I couldn't relate to lot of the behaviours associated with Fi such as always knowing how I feel and always sticking up for what I believe in (probably on account of being a 9 on the Enneagram).