Which of the sentences below is correct and why?
I'm not a loser or pessimist.
I'm not a loser or a pessimist.
It depends on what you are trying to say.
I'm not a loser or a pessimist.
The indefinite article in front of both objects means the sentence should unequivocally be interpreted as
I am not a loser and I am also not a pessimist.
in which 'loser' and 'pessimist' are separate objects, each with different nuances.
In this context listeners might understand
I'm not a loser or pessimist.
to be the same as above. However it could be interpreted as
A pessimist is a loser, and I am not that.
in which 'pessimist' is a parenthetical phrase used to describe what you mean by 'loser'.