I have the same problem, I can't seem to get a grasp on the things I want to say, especially in social situations. I don't know if it's some kind of social anxiety or social phobia or something, but what helps me is I take just a few seconds to "rehearse" what I want to say. I usually say "um..." think about it for a second then answer. I also keep things short and sweet so I don't overthink things. Hope this helps.