If you're going to flame out, might as well be on a practice interview. Don't beat yourself up if you weren't seriously going for this job. Seriously... DO NOT BEAT YOURSELF UP! What happened when it went to shit? If you had a do over, what would you do differently. Please, try to take the positives away from the interview. Mine the experience for whatever value you can get from it. Cut yourself some slack, and lose terms like incompetent from your self image.