Once upon a time, in land far far away there was a princess who read through all the threads the NT's wrote, taking notes as she went along in her pretty pink notebook. With unicorns on it. Yeah! Glittery and unicorns! She cackled with glee! Oh, dear lord... I would drive you guys batty. I don't really write like that. Well, sometimes I do. It's called a false start. I free write and whatever comes out is what's on the paper.
I think the X in your (INFX) type is a P. Just a hunch.
I am an ENTP (with a few years of experience doing graduate and research work and academia) and I have almost the exact same "problem" as you do, my responses are always way over in terms of word-count and people tell me to prune, but the loss of sentences starts to feel like I am giving up too much content.
In my professional career prior to school, there was no doubt that many people found my e-mails and other documents incredibly long ... and I would often spend far more time trying to prune them than actually writing them.
This is still a challenge for me because in many cases, there would be a subset of people who would write to me and provide the highest praise about how what I wrote was "incredibly inspiring" or "the best I had ever read" or told me that I provided details they could not get elsewhere. I would get encouraged to continue on.
On the other hand, particularly with people who don't know me yet (e.g. new co-workers or bosses on the job) and with strong J types (seemingly), my writings would sometimes raise frustration or people simply would not read them.
Moral of the story (for me, so far): Write longest to people who know and respect what you are going to say ... and if you write something very long, make sure it is DAMN GOOD and no one will care.
When this can't happen, I end up just forcing myself to arbitrarily delete huge chunks of text and then try to reconstruct parts with self-imposed limits ... it works, but I hate it.
Can write a paragraph of 15-20 lines which (it turns out) is a single sentence with various nested brackets - I have been known to reach 3 or even 4 'deep' of nested brackets before now.
Also FYI: I'm an ENTP and do the same thing, except with lots of time and discipline I think I am down to 7-8 lines per sentence. I definitely feel that there are cases where there is some threshold (maybe 4-5 lines) below which it is impossible to actually convey the true feeling and meaning that I am going for in a sentence.
On the other hand, only some people seem to actually be able to pull all of the meaning from those long sentences that is actually there anyway ... so it ends up being more about my placating myself and feeling like I actually conveyed what I meant in a way that can't be misconstrued.