I assume from your being here that you've exhausted other methods for determining whether you're INFJ or INFP. I take for granted you've already discovered how to translate these two type codes, and you know in theory what the letters represent. I expect you're familiar by now with Jung's premise underlying all this stuff, and you've delved into Temperament and feel entirely confident that you're a Catalyst. I daresay you know plenty about Keirsey, and Isabel & Katherine, and the MBTIŽ. Hopefully you've learnt the instrument is not interchangeable with the theory and the "test" is fallible and the scores are nearly useless, especially when your J/P score wanders all over the map. And it's not enough. You still can't figure yourself out. So here you are.
(If you have not done these things, I suggest you visit other Type websites first and immerse yourself. Read a book or two -- there are plenty to choose from. Join a type list. Take a free temperament assessment. Hire me to teach you privately. Come back when you're ready for different and advanced stuff. Otherwise, much of what's written here will go right over your head and won't help you sort anything.)
If you are waving a white flag and you've come here with an open mind, perhaps I can offer you some new clues. And don't be ashamed -- you're not the first person to ask!
To better help you distinguish between INFJ and INFP, I'll rely on the Interstrength MethodTM and utilize "triangulation" with multiple models. Your best-fit type lies where the models agree most often.
On this site I'll touch on Temperament and I'll introduce a new model (Interaction Styles -- the Berens version of the DiSC). I also investigate the Type model more deeply than other methods do (Cognitive Dynamics). I'm NOT going to teach these models in depth, mind you -- Interstrength Associates is where you go for that and to learn these models as a whole system. But I will draw upon all three models to offer clues which might help you sort between just these two patterns: INFJ vs. INFP.
One caveat: I'll use myself as an example a lot -- not because I think I'm the quintessential INFJ (I'm not!), but because Catalysts tend to get more out of anecdotal information than cold, hard facts alone, and I can share my own anecdotes better than any others. So forgive me if much of the following seems egocentric or biased toward me.
We have lots to cover, so let's get going!