This tragic condition has come to be known as the "brain fart". It's akward when it happens in conversation, but even more frustrating when you're writing. Some days your mind is so ready to write, and there are ideas pounding in your head, and as soon as you read over what you've just written, you wonder who wrote it. It certainly isn't the mental narration you pictured when you started.
I went through a really annoying period where everything I wrote turned out to be NOTHING like what I imagined it to be. I couldn't figure out what was going on, so I did what I always do when I can't figure something out. I took out the notebook I always keep in my pocket, (which you might consider doing too, by the way, and soon you'll go crazy if you're without it) and I started noting the things I was doing each day, and the results of my writing the day following. I found I most often have difficulties saying what I want to say after days I don't exercise.
In order to test my new theory, whenever I started writing and hit that annoying point where I wasn't saying things right, I went outside and tossed an old pair of truck tires around for a while. They were heavy, and I would come in sore, but without fail within a few minutes things were coming out right on the page.
So, in short, here's your mental Beano to relieve brain farts.
It's really that simple. As always, continual effort is what's necessary to succeed in writing or any endeavor. Just don't give up!