You might want to check something as simple as the speed you're riding. When I first got my road bike, I was exhausted compared to how I felt riding my hybrid. Then I compared the average speeds: I was averaging 9 mph on my hybrid and 15 mph on my road bike. No wonder I was more tired: I was pushing myself a lot harder. I still have to make a conscious effort on my road bike to pace myself on longer rides; otherwise, I get tired too quickly.