For example, you wrote this comment. It is a well written comment. You likely have some belief roughly along the lines of "I write reasonably well". You have probably received feedback along those lines throughout life, and that reinforces that belief. This is perhaps mildly pleasing or at least seems correct.
Now suppose instead that whenever you wrote things people replied:
"Huh?"
"What? This makes no sense"
"Good god what led you to think that? That's so stupid!"
And so on. And you even reread some of your own writings that you thought were good and they seem to strike you as not good. The view that your writing is not good comes to strike you as correct. You aim to improve but continue to receive negative feedback. The view that you are genuinely not good at writing seems to be correct and you come to believe you cannot ever be good at writing.
That would be negative self worth. Does this example line up at all with any internal thought processed you have? Are you pleased by praise or hurt by criticism? Even to the level of thinking the judgements are correct or incorrect.
(To be clear you are good at writing)