I have the same problem. In normal conversation I know the words i want to say but i just cant say it. It feels like my diaphragm freezes and I'm not breathing. I use a lot of filler words like uh or anything else I'm able to say to navigate around the word/words I cant and feel like an idiot. This doesn't happen nearly as often when I'm with someone I'm comfortable around, but will happen at some point if I have to explain or tell a story for a long period of time. It makes me feel socially anxious and I avoid talking even when I'd like to start up a conversation.