Why do I do that?  Why is it that, when somebody I work with compliments me on what I'm doing, my first, automatic, almost unthinking reaction, is to inwardly assume that they're being sarcastic?  I behave as I should, of course: I smile, I thank them, I do the modesty thing, but all the time I'm thinking, "are they actually taking the piss? Is this their way of telling me I'm rubbish?" 
Why can't I, instead, allow myself to take their words at face value?

It used to happen when I worked behind a desk, and it still happens now that I'm pursuing my new path.  Of course, there have been jobs I've done in the past, that I genuinely was rubbish at. I knew it, my employers knew it.  Maybe that's where it springs from...  although generally in those jobs, the folks in charge left me in little doubt as to my lack of ability!