That definitely sounds like a very mild but treatable form of language disorder to me. It might be too mild to be of interest to language disorder clinicians, but maybe you could try 'treat' yourself by setting up a long-term plan of progressively 'harder'/longer reading materials. Check out for instance Pearson's Developmental English series [1]. Might seem arduous and pointless; but if it increases your ability to read long-form comfortably, it could make an enormous difference to your life.

[1] http://www.pearsonhighered.com/educator/course/Advanced-Read...