I don't race, but I find a similar thing with normal riding. In the spring, I can put up with lots of discomfort and hard miles to get fitter, but come summer (like you, about 3 months after starting to ride hard) it's like my body thinks that because it put all that effort in during the spring, everything should be easy, and it it isn't easy, it can't be bothered. As though riding up hills has become a chore rather than a challenge. Sometimes this psychological barrier goes by itself, but I've found the best way is to do some different kind of riding, or have a complete break for a week or so until you're desperate to get back on the bike because you miss it so much.