My experience is that I generally have some kind of fuzzy idea of what I'm going to say before I say it. When I actually speak, sometimes it comes out as a coherent and streamlined sentence whose contents I figure out as a I speak it. At other times - particularly if I'm feeling nervous, or trying to communicate a complicated concept that I haven't expressed in speech before - my fuzzy idea seems to disintegrate at the moment I start talking, and even if I had carefully rehearsed a line many times in my mind, I forget most of it. Out comes either what feels to me like an incoherent jumble, or a lot of "umm, no, wait".

Writing feels a lot easier, possibly because I have the stuff-that-I've-already-written right in front of me and I only need to keep the stuff that I'm about to say in memory, instead of also needing to constantly remind myself about what I've said so far.

ETA: Here's an earlier explanation of how writing sometimes feels like to me.