The opposite could be unsure, or uncertain.
Unsure, and uncertain.
unsure means not positive. SO the opposite(antonym) is... (bum Bum BUM) positive.
unsureIt should be 'unconfident'.Uncertain, unsure...
unsure, doubtful or dubious, uncertain, hesitant.
The opposite of self-confident could be unsure, uncertain, timid, or self-doubting.
The opposite polarity or charge is negative. The opposite of a numeral or balance is also negative, or deficient. The opposite of (being) positive would be unsure or uncertain.
An antonym would be, "I was uncertain/unsure to ask help on the assignment." Both words are the opposite of certain.