Am I too late to join this party? This will be my first reply so here goes:
I'm marrying an ISTP in a few months, and let me tell you I have never been this excited for ANYTHING! Personally there's nearly nothing that I want out of a relationship that my ISTP can't offer me. He's fun and breaks me down when I've been too serious for too long, he's spontaneous, laid back, popular, gets along with everyone, never judges anyone, is hard working, intelligent, resourceful, and wants nothing more than for me to be myself, something that almost no one else in my life has ever wanted me to be! An ISTP won't try to control or change you. ISTPs are So. Utterly. Cool... but they're dorky and goofy at the same time.
Falling in love with an ISTP (the first time) will allow you to fall in love with yourself all over again. A lot of INFJs have lived in a shadow of widespread disapproval about who they really are since toddler-hood. No one wants to hear about how passionate we are for causes that they don't care about, so we learn to stay quiet. No one wants to believe us when we predict how their families will react to their decisions, so we stop warning them. But an ISTP will never scoff at you, and when he sees you with your guard down he will adore that person you've been hiding all this time.
Be warned, it seems like almost every INFJ/ISTP relationship faces something of a reckoning. This "reckoning" can begin at two months or at two years, and it won't be fun for either of you. Your ISTP is most likely not as in love with the idea of commitment as he is in love with you, and after infatuation wears off he will probably distance himself either openly or emotionally, and he may even try to hide it from you. You'll catch on and most likely panic, but when you get that under control the negative cycle will already be in the works. Eventually you will both be faced with a lot of difficult decisions. For the relationship to work, he will have to decide what it is he wants and what it is he feels (not so easy for someone who neglects and doesn't trust his emotions). You will most likely be hurt, especially since the ISTPs weakness here is that he doesn't understand why you have so many intense feelings and why you let them guide you in life. You'll have to be strong enough to forgive him for being insensitive and probably even for lying. An unhealthy ISTP is very self destructive, and he may ruin everything after the explosion of his months or years worth of bottled up emotions. That seems to be the prevalent theme in these relationships, but believe me this painful battle is more than worth it.
You and your ISTP will fall in love again with a new vigor and new respect for one another that comes from fighting in the trenches against yourselves. He will have learned to open up, to regard how you feel, and to avoid any action that could hurt you so badly again. You will have learned to watch for the signs of him feeling trapped. You'll learn to give him freedom without fearing that he will run with it. You'll also realize that he's more observant and expressive than the world gives ISTPs credit for, and he's not shallow like the world (especially NFs) claim he is.
Best wishes to you, I really hope that your INFJ/ISTP relationship thrives. You can be so good for one another if you don't give up.
