If you focus on rolling too much, you may not be able to do so smoothly thus spend to much time being prone.
The sensation you should be looking for is almost like its accidental. Something like, oops I tripped! One hand lunges forward, head down, body rounded and you fall down, only to have your legs go over you and lever your body upwards again. Leaving you surprised and bewildered but not tired or breathless.
And for now, don't do rolls continously. One by one till you have it down pat.