Why not show all the times she patiently listens while I struggle to articulate? Well, because a story is about conflict. As a storyteller, that is naturally what one is drawn to. Besides, although it may not seem like it, I'm really doing her a favor.
In addition to being true, her moment of impatience has the advantage of making her a more sympathetic character. First, it's hard to identify with a saint. Second, a person who perseveres in spite of anger or fear or whatever weakness you name, is more human and likeable. It makes for a better story because it's true. But nobody, including me, wants to be reminded of times when they behaved in a way they didn't like.
As the teller of the story you cannot include everything. You can and must decide what to leave in and what to take out, what to draw and how to draw it. The temptation to juice things up or gloss over what you do not wish to confront is powerful. How well you avoid that temptation will be a large measure of the success and value of the work to the reader. But when it makes a loved one upset, it can't be blamed on a disease.
A note, spell check is underlining nearly every word in the draft of this post. It's clearly on the fritz. I apologize for any spelling atrocities that I let slip.