I often have it happen where I forget what I'm saying part way through a sentence.... sometimes I'm just way ahead of myself and forget where in the story I am. Sometimes I just don't have a clue what it was I was planning on saying.

Fortunately, everyone I know seems to accept this as just being a part of me as I do it QUITE a lot. That or I seem to pass it off smoothly. (sometimes when it happens when I'm teaching I'll say it's because X was just doing Y when they should have been listening instead - which is always something true! haha) Most of the time I just go "umm yeah, lost my train of thought here, what were we talking about?!" and work my way back through the conversation to remember what it was I planned on saying.

I'm sorry that it's bringing you down a lot Mr. Radio. Have you tried spending time in smaller social situations, with like just one or two people who you trust? If not... try it. Try letting them know (even online or through a text!) that this has been going on. Sometimes people will surprise you and be super patient (or will just carry on the conversation!)