The answer is not the word humble nor any derivation thereof. Arrogant being negative epithet has a positive counterpart of humble but I'm looking for the negative opposite, instead.
Arrogant conduct is assuming that ones success is the result of ones excellence and failing to consider that it might be subject to advantageous circumstances.
The opposite of that would be failing to see that any of the progress is due to ones skills nor competence. And that's a negative concept as well.
I can express it by overly humble or excessively timid/self-critical but I want a single word term for precisely that, rather than a description of the property. Insecure won't do because that's a different concept - I'm talking about someone who too often assumes that the success is due to fortunate circumstances.