The most helpful thing to understand about INFP is that they are first and foremost deeply introverted. They are also incredibly sensitive. Some vast majority of their cognitive energy is spent simply feeling their feelings.
And they have so.many.feelings.
This is actually quite a lot of work on an energetic level, and can be exhausting for the INFP, particularly for INFPs who haven't developed skill or comfort expressing themselves, because then basically their whole life is happening inside their heads. The INFP is private and guarded naturally, so even the well-developed INFP is often reticent to express their deepest feelings. When they do, it would behoove their partner to lean in, get comfortable, and listen carefully. The INFP will clam right back up if they feel remotely judged, criticized, or unheard, and it will be a long while before they try again with you. (If they ever do).
It is not uncommon to find INFPs who habitually isolate themselves or push people away, often because they had negative relationship experiences in the past or even just because they fear negative experiences. (Remember they are extremely sensitive, so deep emotional wounds can take a long time to heal). A wounded INFP will protect that rich internal landscape with the tenacity of a pack of junkyard dogs. Admittedly, these self-protective INFPs are hard to build relationships with, but it is not impossible with patience, time, and practical affection.
And I will add here that it is entirely worth it, because any INFP is a wonderful partner when you really snag one. They will give you all the space and understanding you need, a quality that is especially valuable if you're a big, weird extravert who struggles to find a partner who accepts you fully. An INFP will embrace your idiosyncrasies willingly; in fact, they tend to prefer "weird" or unconventional people. And once they trust you, they actually like it when you encourage them to get outside of themselves. They can be hilarious, silly, fun, playful partners. They can be wonderful performers and storytellers, in part due to their rich imaginations. They are also great with kids, in part because they are somewhat childlike and innocent themselves. They do well with bright, optimistic, cheerful, peaceful, steady, independent partners who are undaunted by their dark spells.
Because INFPs (even under the best of circumstances) are prone to dark spells, depression, and self-pity. They are very hard on themselves and prone to feeling guilty or ashamed and getting stuck in cycles of feeling this way. And they will usually withdraw when they are hurt or overwhelmed. It's just how they are. So a partner of an INFP has to learn to curb the urge to draw them out forcefully, because it won't work. They will actually be most likely to dig their heels in and retreat further if you try to force them to open up when they don't want to or aren't ready. That said, they do need your encouragement to come back from the dark side, especially if you have done or said something to contribute to their shutdown. You will need to gently inquire as to what's going on with them, and what you can do or say to make it better, and wait patiently until they are ready to talk to you about it.
I know that sounds confusing.
Here's another way to say it:
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Update: Here is some advice for the INFP him/herself: