You took a severe blow today. I´m sorry for you. It happens now and then too all of us. It seems to me that you are a stutter but don´t have the common repetition patterns that characterize stutters. You just block. The people that has the repetition pattern, they do block too. They try to push the block and that´s why it looks like a repetition. They are trying to pass through a closed path and they keep bouncing. For what you describe, next time don´t try to practice whatever you want to said in your mind. You don´t have too because you already know the answer. It only make´s you more anxious and re-afirm your stuttering mindset. Instead, try to concentrate on the moment, on the Q&A of the others people. You will feel a big rush of anxiety and anticipation for sure. Don´t fight it just let it be. Feel it and let it flow. God bless you and keep on.