Sometimes you might feel underestimated as a copywriter. Consider the person who makes you feel so. If you think he is himself brilliant at his work, chances are he can sense your better hidden potential and just wants you to improve. If the 'gyaan giver' himself has created mediocre work, simply ignore / prove your mettle and move on. Last, but not the least, as long as you believe in your skills, don't care much about those who always love to show others down. Never underestimate yourself and soon no one will underestimate you.
P.S. Writer's block happens and fades away. Keep browsing interesting work and just move on.