This might not be the most delicate thing to say, but
  maybe you feel this way because it's true?

A cold, hard slap from reality wouldn't be the worst thing to receive- it'd at least give me a point of reference to define my abilities.

  Try a controlled experiment. Do something on your own,
  so you know it's all you.

This is so bewilderingly straightforward I can't believe I didn't think of it sooner. Thanks.