A huge difficulty in my life has been that I know exactly what I want, but exactly what I want isn't an option. Which means I have to choose between varying areas and degrees of dissatisfaction. And for me, there are no varying degrees of dissatisfaction--they're all The Worst. Sucks for me. And I guess it's not very nice for those around me. I'm okay at compromise when it's based on an ideal I value, such as compromising for the sake of a relationship. But a big part of me feels it's weak and dishonorable to compromise for the sake of convenience. Perfection should be pursued.
For example, I'm a horrible shopping companion because I have to look at every single possibility in every possible store, and about a dozen items will have three good features and seven bad ones, so I must decide between which three good features I value the most and which seven bad features I can live with. And all that is NOT what I had in mind when I made the decision to shop for item X. Now I must reevaluate everything in relation to all ten possible features of item X, and then return to shop again on a later day. So I go home unhappy and empty handed because I failed to find exactly what I wanted, and I couldn't persuade myself to settle for anything less. When my shopping buddy thinks, "She just didn't know what she wanted," she's right to a certain extent: I didn't know which flawed thing I wanted because I didn't want ANY flawed thing!
I didn't settle on a career until I was 35--not because I didn't know what I wanted, but because I didn't know how to find what I wanted. I often think I'd be much happier if I didn't have such a clear vision of what I want. I always know what I want. But I never know how to give up on the ideal in my head when what I want seems unattainable.
In my opinion, it's not that INFJs don't know what they want but they don't know how to want something because the "how" is subservient to the why they should want it in the first place. The "why", of course, is defined by the ideal that they have set for themselves.
There might be some truth in this. I'm not very good at making flighty, whimsical decisions. I often wonder why people imagine they can convince me that I want something different, since the decision to want that thing is so weighty in the first place. How can they imagine that their paltry reasons can compare with all that lies behind the wanting of this thing?
This is not an uncommon trait in my family. We just call it stubbornness. We have a family story about an old ancestor who was sawing firewood to a particular length. The end of one piece was going to come just to the center of a knot, which would be very difficult to saw through. His friend suggested he cut the piece either longer or shorter, to avoid the knot. The old man growled, "I never go around a knot!" That's me, making things difficult because I can't bring myself to accept a "lesser" alternative.