Fi being misunderstood is an understatement of a lifetime. Almost everything online about Fi is at least partially incorrect. Even google overview's explanation of Fi leaves a lot to be desired; It says that having morals in everyone else comes from other people, which is just...not how this works.
As an INTP who went through a phase of questioning whether I was INFP or not, I came to the conclusion that no, I am not, but I'd be forgiven for thinking so because I have significant moral OCD and if you took what everyone online says about Fi seriously, you'd think they're the same thing. (Pro tip: They're not.)
So what is Fi if not constantly scanning your environment for Bad Things and rejecting all that doesn't make you feel good on the inside?
Well, simply put, moral OCD provides anxiety, Fi provides clarity. Fi is a guide that INFPs (and others) use to figure out what feels right to them in every facet of life. Whether this is used immaturely or maturely entirely depends on the INFP; both terrible and great things can come from this. However, it doesn't primarily come from an overwhelming sense of fear or a desperate need to prevent something bad from happening (unless in dire circumstances, of course), it comes from a sense of catering to one's true self. It's not inherently about catering to one's own morals, though it often utilizes that quite a bit. Rather, it's about one's identity.
Let's run that back again: Fi is about catering to one's identity, not one's morals. There is strong overlap, but identity comes first. They move about the world in a way that caters to their individual identity so as to provide clarity to themselves.
Wanting to be authentic and having morals are not monopolized by Fi users. In real life, virtually everyone will have a point (or many) where they cater to their ideas of themself, become more of an individual, stand up for their morals, and even cry or gasp in horror at someone crossing a moral line. The difference is, this is a 24/7 process for INFPs, and while they can absolutely feel stressed out and anxious about the state of the world (same girl, same), it's a process that works to provide them clarity. That's the important word here. Clarity about themselves and their identity.
While nothing is ever black and white, in general, Fi would look more like:
Examining whether things are right for you automatically, rejecting social norms one doesn't understand simply because they don't feel right, wanting a life that feels like you to an extreme or significant level (note: this is not the same as wanting to be an individual), doing things because they feel right to you, not because they're for the greater good (though you may take that into account as well), life is about building your identity.
Whereas moral OCD looks like:
Constant, anxious preoccupation with doing the right thing, intense fear about being a horrible evil person if you so much as breathe wrong, feeling like the world's problems are on your shoulders, constantly fretting about how much active harm you're causing by simply existing, extreme and intense constant rumination on past and present mistakes, feeling sick to your stomach when you make an error, life is about constantly trying to resolve the extreme anxiety and stress you get from the reality that you might not always be right or say the right thing or be a good person.
Building and catering to one's identity is at the forefront of Fi users's minds. At the forefront of someone with moral OCD is nothing but intense fear and anxiety about the what ifs of everything.
Now, again, everyone will want to build their identity at some point. It might even be at the forefront of your mind for some time. The difference is, it never really leaves Fi users forever. I'm INTP, and I definitely care about my identity nowadays, but that's partially because I didn't really have one at all until I was in my late 20s, and even then it was still blossoming. I had my own morals and convictions and sense of right and wrong, but I didn't really know who I was outside of that, I only knew what I believed, my likes and dislikes, and some traits. "But isn't that your identity?" Well...no, not quite. Look, I'm struggling to put into words the difference, but the best way I can explain it is that an INFP can easily and readily see who they are beyond and beneath that; I can see patterns, I know why I like and dislike certain things, and I know who I want to be, but I'm still unsure to this day of who I am beneath all of that. I don't know. Maybe a broken and scared child, maybe an alien, maybe a chunk of animated meat. I don't know. Generally by my age, INFPs know. There is little uncertainty.
I am defined by my uncertainty; they are defined by their clarity.
