改变心理 (changing psychology)

I've been developing a trading strategy based on trends and news announcment since May of 2006. I've come up with a REALLY good strategy. It is in a constant development still but I keep improving it and theoretically I could make over 1000 pips certain months. Of course theory and practice rarely match up. But here is what bothers me. I tested the strategy all the way back to January 2006 and everything lays out perfectly. I rarely lose. On average maybe 1.5 trades. My loses are limited to 60 pips. I let my winners run. I trade GBP, CHF and EUR vs USD. I have already funded my live account and ready to trade live. But a few days ago I decided to test it abit further just to make sure. I went back all the way to July of 2005. And I noticed that my strategy pretty much fails every month. I had to augment it quite abit to make theoretical profit in a month that I could have made in a week in 2006. How could this be? How could one strategy work so well for 11 months but then just stop?