WTHWIT?
What The Hell Was I Thinking?? The "smack yourself upside the head" moment when you realize that you have made a massive mistake that will take hours, days, months, years or possibly even a LIFETIME to recover from.

May apply to a hair cut, a binge episode involving food, alcohol or chocolate, a career choice, a one-night stand or, more commonly, a life partner.
WTHWIT? I can't stand his sorry ass and yet I spent the night with him? Oh, God.

WTHWIT? I spent my entire paycheck on a hot pair of shoes. I will have to wear them as I walk to work since my car will now be repossessed. Oh, God.