Having worked for another one of the Big 5 and departed, I'm somewhat sympathetic. My advice would be:
-Consider that you might be experiencing the early symptoms of burnout, depression, some form of anxiety disorder, etc. Checklists of symptoms are available on the internet; if you feel you match those symptoms, consider professional help. As as aside, the stigma is not as great as it used to be and, being employed by the Big 5, you should have access to relatively generous medical benefits and a HR department that is conscientious about giving accommodations for health issues (if for no other reason than avoiding bad publicity and an aversion to lawsuits). If you do go this route, I recommend seeking out a teaching hospital, if there's one nearby; you have an improved chance of getting good care there.
-It's time to start leaning on your professional network. Start contacting past co-workers and friends outside your current employer and get advice, do practice sessions, and job leads.
-Technical interviewing is a learnable skill and, as often noted on HN, often has little to do with your day-to-day job, even if you're at a top company. There are books and sites for both the behavorial side and the technical side of interviews. Start drilling an hour every other day until you have your behavioral answers and your techniques down pat.
-Lastly, remember that passing an interview is mostly a matter of luck. Sometimes you just get some git whose favorite interview question happens to be something you didn't study (see Yegge's description of the interview anti-loop [1]). Sometimes you get someone who got up on the wrong side of the bed. Keep trying, learn from your mistakes, and you're bound to find a position.
Good luck!
[1] http://steve-yegge.blogspot.com/2008/03/get-that-job-at-goog...