Did I make a mistake?

In the heat of feeling angry with my partner, I wrote in my phone some very nasty things that:
1. Were not true.
2. I would have NEVER said to his face.

He used my phone (to call his because apparently he lost it) while I was in the shower and saw the things I wrote privately to myself.
He packed his bag and left.

Am I in the wrong for writing those terrible things? He won't believe me when I tell him they're not true.
Did I make a mistake?
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