I just don't want to go down, period.
Not to beat a dead horse, but here's the issue. And there are two ways of looking at in.
One--don't be in the market at all. There is a risk premium because there is risk. That's what gets you your rate of return over the savings rate. Sometimes up, sometimes down, more often up over time.
Two--unless you invested in the market for the first time in Summer 2015, the market has never lost money. It's just a question of how long you held it for.
The reason market timers don't beat the market is because they time the market wrong--they buy high and sell low. Yes, the market is probably going to go down, but when will it go up for good? Did you catch the 6% bump last week? Did you go all in in March 2009 on the way to doubling your money? March 2010 and make about 80%? March 2011 and make about 60%? March 2012 and make about 30%?
It's hard to stomach the volatility, but that's why the market gives the return it does. And why there is a risk premium--because people can't take it and bail at the wrong time.
I say this like it's easy, but it's not. Behavioral economics takes people down--repeatedly. My secret for making several hundred thousand in the market since 2002? Max the retirement accounts and know that market risk over long, long periods is almost nil. And the very first $5,000 I put in a non-retirement account I had to convince myself that my life probably wouldn't come down to $5,000. Even if I lost every penny, I would be fine. And almost every pay check since--I made the decision about some portion of it, and into the market it went.
I have been crazy conservative with my money. I have paid off all my debts. I have paid off my house. I knew that I couldn't stomach the huge market drop if it meant I suddenly couldn't pay off my mortgage. That's not economically rational, but it addressed the behavioral weakness that I knew would exist. Once that was taken care of, I maxed every penny into the market knowing that it was the most rational approach, and that being willing to lose every penny is what would make me one of those rare investors--the ones who don't time the market, and who win as a result.
Btw, I still don't trust myself, so I have everything set up to contribute automatically. My strength is knowing my weakness, and accounting for it.