I have the same problem, usually because when I stop to consider my point and choose which words to use next, the other person frequently takes that opportunity to jump in with more of their own thoughts. Sometimes I think they may not have been listening to me at all, but simply waiting for more air time for themselves. Had a few bosses like that - they'd ask me something, then while I'm answering, they'd cut me off with something else. Very frustrating.
On the boss front, I'm fortunate right now. When I really need to talk, my boss will listen. On the other hand, he's comfortable with my writing, and when I write something, he has good documentation for the files. Even when I come to his office to report on something, we both expect him to do most of the talking. I get a good grasp of what he needs and return to my office to do the documentation. Sometimes, I can influence things a great deal by how I write the findings.