I want to tell someone they have been “difficult to work with” in writing, but I don’t want to put it quite so directly.
Is there another way to write it so that doesn’t sound as if it were some behavioral issue? I am trying to think of words to lighten it up so as not to affect a working business relationship.
Also, the person could be going through a stressful time outside work, and I don’t want to offend.
If there is a reason you absolutely must tell them, be sure to precede it with something positive (and if there isn't a reason you absolutely must tell them, you probably shouldn't). Something along the lines of I always appreciate a challenge, and you never fail to provide one.
You could also just go with a straight euphemism, such as working with you has really helped me grow as a person.
I'd use a sentence like the following where you don't underline only the difficulty but a possible positive aspect too.
It has been challenging to work with you but very constructive also.
My suggestion would be to not give the person an abstract personal characterization. Instead, speak specifically about a particular, concrete action that the person takes, at the moment s?he takes it.
For example, if s?he blows smoke in your face then say that you don't like that -- at the moment when your face is engulfed in smoke.
It is far more helpful, IMHO, to point out particular actions that interfere with your working relationship (in your opinion) than it is to simply express your frustration generally, characterizing the person as difficult to work with. And this is true regardless of how much you sugar-coat things.
It is constructive to point specifically to what the person does (particular actions) that you feel interfere with your working together. And it is helpful to do that when the action is fresh. This is fair to the other person: it gives your colleague an opportunity to discuss the interaction specifically.
You might even be surprised to learn that there is another side to things... It takes (at least) two to work well together.
That kind of progress rarely happens when one person just labels the other uncooperative or whatever. At best, such labeling might lead to a constructive discussion of particular past interactions, but such discussion is more fruitful if it takes place fresh and not a month or a year after the fact.
(This is the same approach you might take with your pet dog, BTW. You would not expect your dog to get it, if you tried to complain about its behavior in a general way and long after an incident.)
And yes, my answer has little or nothing to do with English language & usage. I still offer it. Don't just look for a word to characterize someone or something you dislike. Communicate using specifics instead. Maybe you will then have questions about terminology for specific actions, and maybe then I'll have something more EL&U-y to offer as an answer.
This doesn't directly refer to their uncooperative behaviour and so is perhaps a more tactful way to state your concern.
I have unfortunately/regrettably found our collaboration/cooperation/teamwork/partnership to be an unprofitable/unfruitful one.
It would be best to accompany this with a remark emphasizing the aspects that you've found positive in your relationship.
