Have you ever seen the World War Two-era British poster that reads, "Keep calm and carry on"? Keeping your demeanor calm, even when you're frustrated or angry, is a helpful approach in the long run.
People with moderate-stage dementia tend to mirror the emotional state of the caregiver. If you're happy, she's happy. If you're cross, he's cross.
To help maintain an even keel:
Show your calm in nonverbal ways. Move at a slow, reassuring pace; keep your voice even; use soothing tones; smile.
When frustrated, remove yourself from the room for a minute if you can. Your body language is apt to show your anger even if you can control your words.
Use cues like music, birdsong from an open window, a mug of tea, or other proven soothers to provide a calming environment. Blaring TV and talk radio can make everyone edgy.