"… she ordered me to do some work on a new project that she is obsessed with…"

Having dealt with a difficult person who is now my ex-wife, I understand one basic problem in dealing with a narcissist: most people have an expectation that talking is an attempt at communication. For the narcissist it's an opportunity to impose their agenda.

So for instance, in communicating with a normal person, you might start with, "I'm upset that you were so rude…" and on the other side you would expect a response, “Ah! We are communicating! And I see that you are upset! I feel bad that you are upset! I went too far and I will now offer an apology and also an explanation of why project X seems so important.”

That doesn't work here. A narcissist's assumption starts with, "I don't care if you're upset. You don't matter. What you want doesn't matter. What you think is not worth considering. Only I matter. I should have a salad ‪for dinner tonight‬. My ideas are always correct and better than anyone’s. Why is this person wasting my time when I need to get to the gym? My project is more important than yours." ...and so on.

So, yes, I agree with other posters here: You can't communicate your feelings but you do have to draw a functional boundary and say you had other priorities, and leave it at that.

I have found "Power: Surviving and Thriving after Narcissistic Abuse" by S. Arabi to be helpful in understanding the problem, and I'm sure there are others--Psych Central is great for that too.