It's so hard to say. When a person (whether real or fictional) spends his entire life acting a particular role, it's hard to tell where the actor stops and the real person begins. And that can change, actually. Sometimes you become what you pretend to be, whether for good or ill.
It could be that Dukat was so self-deluding that he never ever let any doubts that he may have had come to the surface - that he kept these so deep that he wasn't even aware that he had them...Kind of the way you describe, Nerys
, except that I have put it far less charitably.
But as Enabrain/PSGarak
points out, it is equally likely that he didn't actually didn't have any, or at least not very often. He never gave me the idea he had any self-esteem issues, you know? He really did seem to consider himself the flower of Cardassian manhood. That enormous vat of self-confidence was one of his most potent weapons.