Oh, something else that comes to mind: do you have any sense of what your non-verbal communication is like? (E.g., how you stand, what you do with your arms/shoulders/hands, tone of voice, the rhythm of your speech, inter-personal distance, etc.)

Sometimes a "blunt and mean" impression in speech can come out of that, as well as choice of words.

Of course, that's less relevant in writing.

The best way I know of to deal with that is to recruit an accomplice to train with... have them tell you when you're making them uncomfortable, then experiment with alternatives, then practice in front of a mirror. It's hard, though; expect it to take some time before you see results.