When to use past tense:
When time is mentioned, such as yesterday, last year, an hour ago, in 1997, etc., past tense must be used.
When to use present perfect tense:
When the event has been completed and the time of the event is not mentioned. "Since" does not tell the time of the event but only from that time onward.
"I just did a stupid thing" is correct, meaning I did the stupid thing just one moment ago.
"I have just done a stupid thing" is also correct, putting the emphasis on the fact that the event has happened. The sentence can mean that the event took place hours or days ago.
When you see 'just', usually we use present perfect tense.
i.e. I have just done a very stupid thing.
It's wrong to write I just 'done' a very stupid thing.
It's because a complete present perfect tense has to include has/have + past participle.
i.e. has/have done but never write done without has/have