“That was mean, I’m sorry” is short enough to text. Most people still don’t send it.

The gap between knowing you were unkind and saying so is not about difficulty. The words take seconds, and everybody knows which words they are. What stops the message is the small drop in standing that comes with admitting it, so the sentence sits in your head for days while the other person keeps the whole of it.

If somebody came to mind just now, send it today, in two lines, with no explanation attached. Leave out the context that makes you look better. A short apology that arrives beats a complete one that stays in drafts.