Oh I was generalising from personal experience too, and I agree very much with your note of caution. These are a bit of a ramble around my thoughts - there's probably stuff here that everyone knows, but it gives some context ......
It seems to be an INFJ thing to empathise with people by bringing them inside using Ni to echo them, faults and all, and so try and help them. I was generalising in the sense that this concern is by no means restricted to narcissistic people, though of course they are drawn to INFJs like wasps to an open honey pot. It's quite dangerous for the INFJ for several reasons inho. One is the risk that they simply echo the other's problems till they become their own problems too. That can happen if the other suffers from deep anxiety, depression, low self-worth - problems that many INFJs have themselves to some extent which is why they sympathise with them so readily – and where my own experiences are rooted. A problem for some INFJs is that they have ill-defined ego boundaries and can lose their sense of personal identity which can become contaminated badly by the other to the point where they lose a grip on who they are. In fact, rather than compromising their inner world ever more deeply, most likely is that INFJs run out of the compassion and emotional energy needed to keep up the empathy for someone who is significantly troubled. This compassion fatigue can take us by surprise – it’s probably a special case of the stress curve with its sudden abrupt performance precipice, and it’s legacy of anger, depression etc. What is even more surprising to us is that it isn’t easy to replace our ‘stock’ of compassion - it can be the emotional equivalent of permanently weakening our back by over-exertion. This can result in sudden changes in INFJ attitude to someone in the outer world, though the problem will have been developing internally for quite a while beforehand. These are extremely difficult issues for us to deal with, because we will have invested a lot of energy in the internalisation of the other – a feature of all our close relationships I think, not just the pathological ones – so we will have made a deep commitment to the other - who themselves may be very dependent on us. The result can be pretty awful both for the INFJ and the person they are trying to love and help, and leave them both traumatised, or at least with significant new emotional problems
My own view is that compassion is not just an instinct – there’s a skill that needs to be learnt too. The way we should learn it is probably the same way we learn to walk, with guidance, with trial and error, and some tumbles and bruises, and it’s best done in late childhood, adolescence and early adulthood. We need to have a clear sight of our inner boundaries, beyond which we do not allow the other to penetrate – after all, we don’t willingly let others stab us or poke us in the eye and we are always damaged by these physical invasions when they do happen, unless they are part of a medical procedure. The mental invasions are as bad, but much less tangible, so we have to learn the hard way where our boundaries are. I was in my 40s before I really appreciated this, and still have some problems with it now 25 years later lol.
The flip from Ni/Fe to Ni/Ti is interesting. Ni is our dominant, not Fe, and we do have to withdraw into it to access it. If we do that in company, it can look a bit like this to others metaphorically. Quite a way away from Fe if it becomes visible to others. But as an INTJ, you must experience something a little similar to this.
Where we are very different from INTJs is that many INFJs, particularly those who have a reasonable amount of Enneagram 5 in us, are pretty good with Ti – you can see that here in the Forum. Because it’s inward facing, I find I can spend all day long journeying around my Ni with my Ti, - in fact I can’t easily access my Ni in ‘humanspeak’ without a lot of Ti to navigate it and translate it into words. It’s cosy too because I don’t need to do any uncomfortable extraverting to evaluate my perceptions. It can float off into unreality though without an extraverted viewpoint to break the loop. What will certainly move me quickly from Fe to Ti in company is if the discussion is complex and I need to dig into my Ni to get at my perceptions. My Ni is like a wide landscape and can’t directly be expressed verbally and linearly so I need Ti to navigate it before I can express it. If I’m dealing with a logical rather than a value-based topic, my Fe isn’t much use, and I’ll respond from the thinking side of things. This post is an example of my use of Ni and Ti together. I have a core intuitive feel for what compassion looks like to me as INFJ, but it isn’t expressed in words within me – I need Ti to linearize and verbalise it for communication. Of course, I’m talking
about it here – I’d use Fe if I was actually
using it in company. Mind you, these words don’t give a brilliant translation of my Ni perception because compassion isn’t really a separate differentiated faculty within me but merges into all my other faculties.
I might well flip to intuitive / analytic even with those close to me, but I'm more likely to bury them in over-analysis and unwanted solutions than appear cold. I must say, there are times that I get fed up with Fe, and it's a real treat to just use Ti instead. But then just as often, I get really fed up with Ti that doesn't know when to shut up (at 3am for instance!!), and it's a blessed relief to use Fe. I do far better in company with Fe than T, unless I'm with people that have a particular purpose that needs it.
Interestingly, I do experience analogous jumps in INTJ behaviour. Some INTJs I’ve known can become very values-oriented where a moment before they were expressing things pretty clinically. Just as INFJs can over-think in public (who me ???), some INTJs can suddenly become very righteous. I had a boss like that many years ago.