Suggestions: Having trouble peaking

I'm in cross country. Every year, I do better and better through the first 2/3 of the season. Then, as soon as we start backing off on the really hard workouts, my times get significantly worse. I went from a 16:41 to a 17:50 this year. I did get my time back to 17:30 by the end of the season, but I couldn't even get close to the 16s again. This has kept me from going to state for two years now. Also, I always feel the benefits of a workout two or three days after instead of the usual two weeks afterwards. Is it possible just to recover and peak really quickly, so that after one easy week, my peaks already gone? What could be other reasons for my inability to peak? Does anyone have any suggestions?