You have almost exactly described high school for me. Everything before, say, grade 10 was a cakewalk. When things started to require actual, you know, work, I crashed and burned. And I was in Ontario, which had dedicated gifted classrooms (at least in grades 5 through 8). As a result, my performance suffered in the last years of high school, which resulted in my going to a community college because I couldn't meet the entrance requirements for Computer Science University degree programs.

It took me a while to figure out how to consistently apply real effort to hard projects. There was always this nagging sense that things should be easier since they were in the past.