Don't allow yourself to be shaken out unless the company really is in truoble, rather than simply going through a slow patch and a period of negative sentiment. I've said before that GSK went through this worry and rotten sentiment a year ago (dredge up the hopelessness from the Fool boards), while VOD were the whipping boy in 2010.
If you sold Tesco, what would you do with the money?
If you think about it logically and ignore your anger/despair/frustration due to the hit to your current holding, is the share you're about to sell actually likely to be your best purchase at current prices?
Investors would be much less bothered if the shares had slipped by 15% over several months or a year, rather than all at once on one day.
In fact, that's how bear markets work; they grind away slowly but persistently, with a few nice rallies to keep the hopes alive, so as not to panic investors until it's too late.