^^..I was getting scared. But it was still a screen pick the next month (June), so I bought it at the monthly rebalance. Up 11% from when I sold it, that I missed out on.^^

I empathize with you and have made the same mistake so many times it can be disheartening. I wrote about this very same thing on the Berkshire Board also where Ben Graham emphasizes the same thing - is was in regard to dollar cost averaging but if he was asked about Mechanical Investing I really think he would have given the same answer because, in summary, it isn't the success of the system that counts so much as whether you can follow it consistently for 10-20 years.

Andromeda replies on One year after Buffett joined Graham

Graham's answer about following dollar cost averaging: "I think there is no doubt, that if you put same amount of money in market year after one has a great chance of coming out ahead regardless of when he begins, and particularly regardless if he should begin now. You have to allow for the human nature factor, that no man can really say definitely know how he's going to behave over the next 10 to 20 years, and there is danger that people start with the idea of being systematic investors over the next 10 to 20 years, and may change their attitude as the market fluctuates. In the first instance, put more money into the market because they become speculators, and secondly get disgusted and scared and don't buy at all later on when prices get low. It's a psychological danger. The fault is not in the stars or in the system, but in ourselves".