Stephan,
You're right, self defense and justification seem to be two of the greatest hindrances to actually taking responsibility.  I think they are difficult to fight against because usually there is some merit to their arguments.  There WERE other causes, there WERE other factors that played a part, and may have influenced us.  But the key for us to remember is that we still had a choice... we still either made a bad decision, chose selfishly, acted poorly somehow to cause the failure.  Only when you and I are able to truly say to OURSELVES, "I did it... it was my failure, regardless of anyone else or anything else... I was wrong, and I have to take responsibility.", only then are we able to move past the failure in our life to find healing, a new start and joy.