Just because you can't tell doesn't mean that it's your fault. There are lots of reasons (notably ASD) that can make it hard to tell even if you aren't a) dealing with text b) from a person you don't know c) communicating in a different style than you're used to.

It'll be easier to improve if you don't blame yourself for things you can't help.

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Honestly, I've had very little luck to get someone to calm down if they are actually mad at me, but I still try to try.

the only path there is to work out what they're mad about and productively, empathetically, and concretely work slowly towards a common ground while genuinely apologizing in specific for offense given. it's much easier when they're not mad directly at you, though, as it's more straightforward to establish empathy.