@aphrodite
Hi Aphrodite, another infj here. Reading your post today reminds me a bit of my own IRL ENFP. I doubt it had anything to do with what you said. Some other person tickled your friend's fancy and off the frisky puppy runs after that new bone!
I've learned that if I wanted to keep this INFJ-ENFP friendship alive, to take it neither too seriously nor too personally and to take it real slow. The ENFP will NEVER be lacking for friends and invitations and will always be off to some great adventure. Then, sure enough he/she will come back to tell you all about it if he/she likes you enough. Prepare to be entertained or prepare to be jealous, depending...Chances are he/she will have told half a dozen people about it already! So, if you want to be that special person in their lives, forget it. They have half a dozen if not more 'special persons' supposedly 'contributing to their personal growth', etc, etc or being their special audience, bleh.
Take the friendship lightly and slowly enough and he/she will pull you along on the next adventure/trip. You'd get to tap into that spontaneous side of you that rarely comes out. Start getting jealous and miffed and your ENFP backs off and you're the poorer for it. Anything that looks like control or confronting them with their fickleness is a turn-off.
Yes, it sometimes feel like we're just teddy bears to be taken out only when the ENFP wants comfort or to have a sounding board for personal issues. Be prepared to have a HUGE heart and an easy-going manner for those big kids! It's up to you to decide whether you think they are worth it or emotionally TOO high-maintenance. You'll get disconcerted by how much of them is 'public' when so much of you is 'private'. A side of them never grows up. I know for a fact. I got two IRL ENFP and do they take lots of emotional energy! One is retired and my own kin and the other is about my age.
So on those off-days when they are happily exploring other friendships and running after this and that, be very GRATEFUL and take the time to RECHARGE. Soon enough they'll come looking for their rock of stability ! (and don't we make good listeners!)... Again the best recipe to NOT feel taken for granted or irritated is to listen with half an ear...
As for them losing interest in you and finding our quieter selves rather dull, never fear. They are attracted to brainy types and INFJ can be brainy. Don't we have many layers (like they say)? Just don't hurry too much in showing them all!!! Learn to make your mysterious side work for you. Also, watch them perk up when suddenly some day you reveal some hidden side of yours after they thought they had you all figured out! They may even complain that they tell you 'so much but you never do'... Take it very slow would be my advice. If they seem to open up fully, don't reciprocate as much. Open up only half-way! NOTHING about themselves is taboo. They self-disclose things that you would blush disclosing! So don't think that when they are opening up, it is such a big deal. They do that with lots of other people, you'd be surprised. But they want all their listeners 'to keep mum about it' (!?)...
Although they seem fickle, they do give you their full attention when they are with you and are storing it up. Careful with those secrets or those quirks. Next time you confront them about something, they will use it against you. Then you'll feel hurt and mope that in this friendship/relationship you're the one who likes/loves/gives more... And that cannot be proven.
Lest I have offended the ENFPs reading this, let me hurry to say that every time I think that I love my ENFPs dearly, both my kin and my friend. But I am not blind and I can see the areas where I have had to work to make the relationship/friendship work. If I had to wait for my butterflies to get grounded, I'd wait forever! So fire on, I am sticking by what I just wrote! Yes there are times when I wonder whether it is really worth it. But then, I have put in a lot of work (emotional energy and all ) in it and don't like to give up on something I have worked on inasmuch as it depends on me.
And oh, Aphrodite, invest of yourself a bit more in some kind of rewarding work, so as to have LESS time for those ENFP. Bury yourself in some work. That always helps next time you feel a bit moody and miffed towards your ENFP and you know that airing it out will not work with these borderline narcissists. Sorry dear, I am really tired, lot of work you know... That should do the trick. Get them to learn to PLAN a bit more some time with you instead of all those impromptu visits and phone calls late at night or first thing in the morning.
Would I doorslam an ENFP? Maybe... I think that I'd tip-toe out of their lives rather than make a big doorslam. Not to spoil their party. I once took a sabbatical from the friendship and it was hardly noticed... That's just to say how tenuous any friendship can be, early days or not...
Good luck Aphrodite.