An antonym (opposite) for certain is maybe not certain
certain
The opposite of doubt is certainty. The opposite of a doubt about a person could be trust.
An antonym would be, "I was uncertain/unsure to ask help on the assignment." Both words are the opposite of certain.
uncertain
Uncertain
Uncertain.
There's no certain opposite, but an alternative would be: Popeye's spinach
Certain, absolute,
certain Confident
because it isn't the opposite
A bird is not limited to fly the opposite of the ocean flow. Birds can fly in every direction but it may be easier to fly certain ways at certain times.