Over at Mind Hacks (yes, I do read other blogs) they mention the Dunning-Kruger effect whereby people incompetent at an activity ae so incompetent that they can't perceive their own incompetence and believe they're quite good at the activity. Wikimedia explains this more lucidly. At the other end of the scale, highly skilled people underrate their ability.
Does this apply to writing?
Well, I haven't hit the highly skilled end of the spectrum, so I can't judge this unnecessary modesty. However, looking back at when I began to write and submit my short stories and first novel--hoo boy. Did I ever over-estimate my ability. As a result I was quietly hurt and frustrated that editors couldn't see and appreciate the brilliance of my writing. True, it had lots of adjectives and a thundercloud of adverbs, I repeated people's names about ten thousand times, and there was often little or no conflict. Oh, and the stories were hackneyed plots. Actually, there were lots of problems. I just couldn't see them.
As you practice any new skill, the flaws in your work gradually reveal themselves to you. And to give editors the credit they deserve, they are generous with kindly criticism and advice that helps to open your eyes. Over at Carina Press, Angela James shared the most common reasons for rejection of romance manuscripts. There are lots of other sites that offer similar writing advice. A Google search will find them. And participation in the discussion at Absolute Write (sidebar link) will restore your dented confidence. Writing is try, try, try again.
Maybe one day we'll all reach the point of undervaluing our writing skills.