Mind going blank?

I have been a member of SCS for three months now. I was so nervous to do a coaching call but I set it up and had my first call last night. I feel proud of myself for making that step. I have always had trouble with thinking “I don’t know what to say in conversations.” I feel my mind goes blank and I become embarrassed. The embarrassment prevents me from putting myself out there. Would this be considered a thought error? It feels so real and there’s evidence of it happening daily when I talk with other people. Are you able to explain why my brain goes blank and what I can do to work on this? I try to think of what I’m going to say in advance but the thoughts become jumbled and I feel frustrated. And when I try to just go with the flow in a conversation it still happens. Any advice would be appreciated.