If you’re not ready to give up just yet, send a thank you note. Thank him for his time and let him know that your speech is not an accurate reflection of your intellect or ability. Then say what you wish you could have said in the moment. No matter what the outcome, doing something like that for closure allows you to reflect on the experience, learn from it and move forward. Maybe bring a similar letter to a future interview that you could hand to the interviewer if you block. It sucks. No way around it. And no magic to fix it. But you are not mentally handicapped. You are cognitively normal and capable of excelling. Keep at it. You will find something that works for you. I believe in you. Believe that until you believe in yourself.