Writers often use idiomatic phrases, and that is fine, but it carries a risk that an editor might presume that you are writing idiomatically out of ignorance of grammatical rules. I actually like to hear regional English, and I incorporate idioms into my writing. But not everyone enjoys reading idiomatic writing, and some editors penalise it.
Sitting/sat. Correct form; ‘I am sitting, I was sitting, he, she, it is/was sitting.’
You say someone is a ‘sitting duck’, not a ‘sat duck’, don’t you? Likewise, ‘I’m sitting pretty’ not ‘I’m sat pretty’ unless you are some kind of anarchist, (my online grammar checker actually indicated that the last example was incorrect).
Writers beware: as an idiom, in conversation, or possibly even in first person narration, it is fine, probably. But it needs setting up and some editors hate it, and have been known to reject manuscripts if it appeared that the author did not know basic grammar, as the habitual use of an idiomatic form may indicate.
The same applies to standing/stood. You talk of ‘standing orders’ not ‘stood orders’.
It is of course correct to say, ‘I sat‘ or ‘I stood’, WITHOUT the verb ‘to be’.
It is quite possible that the ‘sat’ or ‘stood’ form of the present tense participle will become standard in the future. There are indications that this might be happening. But that is something for the future, not for that manuscript about to wing its way to an agent or publisher.
Next time: “Different from/Different to”.