Communication Woes

I have a distinct problem in that I often want to communicate a very specific message but that I also often seem to lack the ability to articulate that message.  This either results in me communicating something I feel does not entirely reflect my intent, or just not saying anything at all in order to avoid mangling what I want to say.  It's really very irritating.  But sometimes, I can communicate exactly what I'm thinking and it just seems so easy.  Why can't I just be able to do that all the time?  I'd like to at least be able to understand better what the difference between the situations is, so I could maybe do something about it.