It's true that if you miss the best 10 days a year your return is miserable, but since those 10 best days are often right after the worst 10 days, your actual results are probably more in line with the fraction of time in the market multiplied by the average market return.

If you feel like you have to sell, give yourself a target of how much further the market goes down (or how long if it doesn't) before you buy back in, and then stick with it.