Imagine a man standing in the terminal with a ticket in his hand, refusing to enter the plane because he feels unworthy to fly! Such an attitude would prove that he doesn't understand the basis for his admittance to the plane. He would probably end up a nervous wreck and never get off the ground. He doesn't understand that his worthiness or unworthiness is not the issue; the ticket is what counts.
Think of the implications. If God (whose standards are far higher than ours) has completely forgiven and accepted us, why can we not accept ourselves? Why can we not believe His verdict in the matter and accept ourselves as He accepts us? If we have received the ticket, which Christ paid for on the cross, we have no need to feel unacceptable.
Edward Lutzer, Failure: The Back Door to Success