Quora answer: What causes people to lose their train of thought (when conversing with other people or presenting an argument)?
Here is a good place to mention something that is normally not thought about which is conversational memory.
We have an amazing capacity to remember everything we ever talked to someone about and to jump to the place we we at on a certain thread left off long ago and to continue from there. This conversational memory is different from both Long and Short term memory, and is almost always social, meaning we can only really remember where we left off conversation threads while we are in conversation with that person with whom we share a conversational memory.
Now what happens to me when I lose a thread of thought in a conversation is that I normally think of something else I want to say, and that knocks both of the threads out of my mind. I think Conversational Memory is conversation thread oriented, so if we try to jump to another thread while we have not yet articulated the one we are on we tend to get lost in the maze of conversational memory that we are keeping in mind as we talk. Notice it is the other person who normally knows what we have just said and they will remind us what thread we were on and then usually we can remember what we were going to say, and then say it. But in that instance we usually forget what we were going to say on the other thread of thought that was being woven into the conversation.
I am not sure why there are so few studies of conversational memory when it almost seems more important than the the other types of memory because it is what keeps us on track not just in our conversations but also perhaps in our lives.