I would say I’ve felt exactly the same way you do. There are several points I feel compelled to make.
I think in terms of writing in general it helps to clarify what it is you want to say before<em/> you write it down- boil it down to a simple principle. Then make your point based on that principle
In terms of ADD, I believe because of the way our minds often formulate several thoughts at once – or so it seems, we may have a harder time in writing it down in a way that is later clear. The impulsive urge to act is hard to stop long enough to think things through.
By the time we are writing our mind has already thought lots of different thoughts.
To be honest, I’ve often written out a long post then deleted most of it because it didn’t stick to the original point or just not sent it at all.
So you are not alone in this Kylep.
Oh and one last thought just popped into my mind as I was about to hit send 
……
Perhaps insecurity about saying what’s on your mind might come into play as well. In face-to-face communication I find it hard to sometimes articulate what i want to communicate because I can’t stop my brain long enough to capture the thought and translate it into speech. As a result I have often said something others thought was ridiculous or argued against logically. I then had no rationale to back up what I was saying, so gave in to whatever the other person was saying or wanted. Over the years I think that kind of plays with your confidence. Sometimes when I write this also takes hold. I figure if I put something down in writing, my perhaps “dumb idea” will be there for all to see for cyber eternity.
Decision making for someone with add can also be difficult , a seemingly simple question when you really analyze the components may seem unrealistically huge before you step back and take a look from a distance. So whatever reason(s) you think you’re post is not well enough crafted may just be simple but figuring them out may not be. Too many thoughts running interference with the ones you should be analyzing to decide if the post is “ready” yet. As may be evidenced by the paragraph I just wrote.
But here’s hoping it’s clear enough to warrant hitting send…..