Something I do pretty often is actually start out with some variation of "I'm not a great conversationalist, but I'm trying to get better, so, there's going to be awkward spots". Or, "sorry, my brain often just freezes up in conversation. I have thought about the things we're talking about, but I can't pull them from the freeze at the moment and it's embarrassing". Or something like that. Maybe that could help. Dunno, it helps me anyhow.