Couples don't need to avoid conflict. They need ways to get through it without causing damage.
Listen to understand
Before you respond, repeat what you heard: "So you felt left out when I made plans without asking?" It slows the conversation down and shows that you're paying attention.
Speak about your experience
"I felt hurt when you canceled" invites a conversation. "You never care" starts a fight. Stick to the specific event and how it affected you.
Take a break before things escalate
If you are too angry to think, say so: "I need twenty minutes, then I want to finish this." Then actually come back to it.
Repair quickly
Own your part, without a "but" on the end. A simple "I was unfair just now, I'm sorry" can change the direction of an argument.
Check in when things are calm
Don't save everything for a crisis. A regular, relaxed talk about how things are going keeps small issues small.