negative/out-of-control FiSi loop: To use a really stupid metaphor, it's like constantly ripping open my own wounds (or picking scabs, depending on the severity). Reliving painful memories (many of them from long-past experiences), futilely looking for some kind of new understanding or closure by looping it over and over in my brain when in reality the best thing to do would probably be to accept that some things will never heal 100% (such is life), but that I should at least let them partially heal by moving forward, engaging with the present, and finding new possibilities (using Ne).
When I'm in FiSi mode, the present and the future don't seem quite as real as the past. It feels like being imprisoned in this gallery of (often exaggerated) negative memories and ideas that I find are somehow related to my core fears about myself and life in general. I don't want to go into detail about my own, but I know an INFP in real life who fears that she is unlovable, and when she goes into FiSi mode she fixates on her past experiences that in her own mind confirm that others find her unlovable while ignoring evidence to the contrary. It's like she fixates on certain parts of her past to form a tragic story about herself, and I can relate to this.
I also get really fixated on routines like an unhealthy ISxJ.
However, I find that FiSi in moderation feels like an "anchor" for me; it's only when it goes on too long that it becomes something ugly.
I relate to this, except for the routines. It's something like a reel of shame/pain & dejection at things ever changing or being different from the past (or even what may be felt as a negative present situation). The idea of things not changing & thus not moving closer to Fi ideals is basically extremely depressing & makes you cynical and morose. Reality is the most disappointing thing imaginable, and in this mode, you easily imagine lots of disappointing things.
I wish I could get stuck in a routine

. I have such trouble with adopting any at all... I will destructively pursue change & chaos rather than get stuck in routine or "sameness". The closest I can get to a routine are "habits", some bad & some good, and mostly involve food/drink or locations I favor for reading or working.
Instead, I get obsessed with collecting information about a topic. This is a substitute for action. Instead of acting with a Ne mentality to suss out & pursue possibilities in real time, I read books & research topics online. I will say that "everything I know about life, I read in a book", so instead of actual experiences informing me, it's Si storehouse of second hand data I've accumulated plus a distorted memory of my own experiences (exaggeratedly negative). I also build lots of theories & predictions (negative). The theories are more helpful, and if they do include enough Ne perception about the workings of reality & Fi grasp of the human condition, then it can be insight backed with some actual research. But these still serve as substitutes for living, something I'll dive into because I do have a negative Fi-Si mentality that reality is pig sty, always has been & always will be.
It's hard to separate this from inferior Te, which is heavily there also, adding to the sullen & morose hopelessness with a hyper-critical attitude (mostly towards yourself, but other less-than-ideal things). This fuels the negative Si experience of the past & even present as being mostly bad. The Si is actually weak, so instead of noticing nuances of change & little details that differentiate, everything looks the same, and that is frustrating & fuels a conviction that things cannot change, so why bother leaving your bedroom. I think it is actually in Fi-Si loops that an INFP may take on more bizarre or extreme behaviors or ideas partly because they are striking enough to feel different, and not more of "the same".
This actually can lead to procrastination & impatience with details, because the sameness of it all makes your eyes glaze over & you find it hard to focus or be motivated to even start; or if you are actually doing some task or project, then you can get obsessed with making certain details perfect & lose sight of the larger purpose.
I am not a very sentimental person, but healthy Si is when I can recall positive things from the past, and also to see usefulness in any kind of repetition, and to not throw the baby out with the bathwater (ruthlessly destroy something imperfect but perhaps with some good or even ability to be refined in favor of seeking "new & better & promising"). It's funny how when I am in more of a Fi-Ne mode that Si thinking is more positive & productive for me.