It sounds like rapidly changing emotions, a.k.a. mood swings, afflict xNFPs the most. xNFJs are affected but not to the same degree. I experience pretty strong mood swings maybe once a week or so. I hate them and wish I could control them, but trying to control them seems to make it worse so I try to just ride it out. It must be really tough to have the emotional rollercoaster even more often. I'm really glad I read this thread though, I think it will help me better understand where xNFPs are coming from.
Quick story -- I am a political junkie and love debating politics. I'm very liberal and my roommate, a male INFP, is very conservative. Before I got to know him better, I would sometimes try to spark debates with him. For me it was fun and lighthearted, but in the middle of one conversation he turned to me with a pained look in his eye and earnestly begged me, "Please don't do this," and I realized the debating caused acute emotional turmoil for him. Perhaps related to the rapidly changing emotions? Of course, after that I laid off the political conversations!
This also makes me think of my best friend from college, a female INFP, and my bestfriend from childhood, another INFP. Both of them were on medication for depression, had substance abuse problems, and were self-cutters. (Both are doing much better now.) I knew they both wrestled with a lot of emotions beneath the surface, but now I'm beginning to understand just how powerful and challenging those emotions must be for them. Is there something I can do for my xNFP friends when intense sad emotions strike? Is it better to leave them alone or try to lighten the mood?