Could it be as simple as that you're not conditioned for sustained effort above a low threshold?
Consider altering your training and mixing in more medium/high effort for longer intervals. Or look for some long shallow hills to climb.
Runners are divided into three classes, sprinters, milers, and marathoners. None can do very well in classes other than their own, so no reason cyclists should be any different.
Find yourself a coach in your cycling group, and train with the specific goal of raising your range at speed.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.