Hi Luke, you definitely have more a of grip on the whole methodology and mechanics of personality typing than I do. I don't even really get the whole Fe Fi thing...it reminds me of the elemental table and I never memorized that either.
I do have to say though, that the ISTJ and ENFP thing -- perhaps that is not going by MBTI but Socionics typing. My long time friend is an ISTJ according to MBTI and HAHAHAHAHAH hell no we could never be in a relationship. Her J gets much more prononuced and rigid as time passes (been a decade) and now she is constantly exasperated by me. That is the exact word. Exasperated. Like an overtaxed mother of a toddler or den mother of an unruly brownie troupe. She is so literal and so linear thinking, she totally puts a damper on my escapades! She even told me I need to see her therapist!!! Just because I told her about my latest romantic fling. I do help her relax and cut loose though.
Well I dunno, maybe you do need to see a therapist.

Si is one trait that helps you memorize lots of variables. In ENFP, it's weak but in the preferred category -- i.e. if you exercise it, or see someone using it, it will make you feel better. Your ISTJ friend is probably exasperated by your failure to listen to what her Si has to say.
IJ types are actually less J than they seem and IP types are less P than they seem. ISTJ however is more S than they seem. J and S stereotypes overlap significantly, so probably that's what's getting to you. Si is very inflexible because the person does not want their physical world to undergo unnecessary change. They instead try to make life more comfortable and cozy. Ne types all crave this, because physical chaos distracts from their ability to make abstract observations.
If you're ENFJ that would make ISTJ your conflicting type. (ISTP would be your dual in that case.) But I think more likely this is a dual you're talking about -- you've been friends for a decade, and it sounds like they are criticizing areas you could benefit from improvement on anyway (and vice versa -- at least duality is symmetric). Out of curiosity, have your relationships with ISTP types been more positive or negative?