For me, reading out loud is so hard for me. Not like voice acting or anything just normally reading. I'm a pretty good reader. I read a grade above the grade I'm in now. But as soon as I open my mouth out loud and read, I start to stutter and stop at random and then I'll even have to figure out the meaning of a word or a whole sentence when I read out loud. But if I stay silent and read in my head I'm fine. For other it seems so easy. Its also not like a nervous thing either, like reading in front of people.
That's just a shyness issue. I have the same problem, even now.
Sometimes certain people aren't good at some things. Introversion and public speaking generally don't meld well together. Not without a lot of practice, at least.