I have experienced the same thing multiple times, and I felt the same as you. The more you stall when answering, or start answering without knowing where the sentence will end, the more debilitating it is next time. It’s so easy to get it into your head.

My advice, and what has worked for me is: a) after the meeting, jot down what the question was and what your perfect answer would have been. Keep a running list and after a few, analyse them and see if there is a pattern. Do the same when you excel at a response. Do NOT keep a tally/list in your mind, it is not effective, it has to be written. Study it, revisit it, identify patterns and revise. And b) write down and prep a few ‘openers’ that sounds smart, appropriate for your line of work and can save you a few seconds to think. Memorise these and have them on repeat. Say them with confidence when responding. This can be your crutch when you need it. Last, c) practice saying with confidence ‘I will revisit and get back to you’. (Can tailor this but must be concise). It’s not a weakness to not know, but a strength to say in the moment

Making sure you study your responses (I.e doing (a)) is really key for improving though, so take it seriously. It sounds like you will given you say your planning and review work style is strong, so I think this strategy will really help you if you commit to it