‘You always cancel’ may express a real frustration, but it invites a debate about the times the person did show up. The practical problem can disappear into a contest over the word always.
Use a bounded description: ‘Our last two plans changed on the day, after I had already left home.’ Then explain the impact: ‘That made it difficult to arrange my evening.’
Finally, make a request connected to the issue: ‘Can we confirm before either of us travels?’ The person can respond to a specific arrangement rather than defend their entire character.
Be accurate about the examples you choose. Do not collect unrelated grievances just to make the pattern look stronger. If you are unsure of the details, say so and focus on the situation you can describe reliably.
The same approach helps at work. ‘The last three updates did not include an owner’ is more useful than ‘Nobody here communicates properly.’
Before raising a repeated problem, write down the event, its effect and the change you want. If the draft depends on an absolute such as always or never, check whether a precise example would communicate the concern more clearly. Specificity can keep a difficult discussion connected to something that can actually change.