One of the most useful half hours I ever spent was listening to a neuroscientist talk about coaching and thinking differently about underperformance. 
I have used it almost every day since to improve my own performance and help other people to improve theirs. And it is simply a matter of understanding some basic science about your body, blood and oxygen.
When you are not performing at your best and you start to feel threatened by it, think about how you feel. What do you notice about how your body reacts to a threat? – do you want to “fight” – perhaps you get tingling/sweaty palms or a flushed face or does your body want to “flight” – perhaps your stomach churns, your heat beats faster or your feet start twitching ?
These are all totally normal reactions. And very a simple physiological level, all these reactions require blood and oxygen to exist. Thus blood and oxygen has to go to the place in your body that is “hosting” your reaction – so if you get a flush on your chest, blood and oxygen have been diverted to your chest in order to make that happen. If your heart is beating faster, more blood and oxygen is diverted to your heart.
The problem with our bodies is that we can’t produce new blood and oxygen quickly enough in order to make those reactions happen without a chain reaction elsewhere. So the blood and oxygen diverted to your heart or your chest need to have come from somewhere else. A part of your body is starved of blood and oxygen temporarily in order to give you that flush or that tingle. And guess which bit of the body is starved ?
Yep. The brain. Bluntly, just when you need your clear sighted thinking the most, you are probably at your most stupid. The particular part of your brain that is affected is the part that deals with logic and problem solving. It is thought that with reduced blood and oxygen levels caused by the threat response, it reducing in productivity by up to 80%. 
It’s an appalling bit of faulty wiring for modern life ! – But given our bodies evolved thousands of years ago and the chance of a re-wiring project happening in our lifetime are zero, then we just have to work with it. It’s probably less time consuming than working hard to somehow “grow up” and get enough experience to not react in this way. Or worse, put up with spending time with people who make us feel under threat in the hope we will get desensitised to these reactions…How about we work on an easier (and nicer !) way to manage it ?
How would it be if we could reduce those times where you berated yourself for blurting out something ridiculous or untrue when you were last in a tight spot at work or were having an argument with your partner? Or thinking later “Why on earth didn’t I say that, I meant to but my mind went blank?” 
Well first, relax a bit and let yourself off the hook. It was because your brain was starved of blood and oxygen. It’s not because you are stupid (well not for more than a second or too anyway!).
So given that this is brain wiring we probably can’t fundamentally change, what can we do ? Here are my top 3 tips:
1) Knowing this science can help. As can paying attention to your body. At least now when I get tingling palms, something in my brain vaguely remembers all of this. I take a deep breath (to replenish some oxygen) and try NOT to speak for a moment. It takes practise but it does work.
2) Remember other people feel this way too. Have you ever put someone on the spot and asked them for their best ideas or lost your rag a bit and then asked someone to explain themselves ? And got nothing? Or a story that made no sense ? Well you know why now. So a smart choice to save time and get better results is to give other people a minute before you get them to respond when the stakes are high and tensions are higher. Or you might spend hours unravelling it afterwards!
3) Use some coaching skills. Adjusting the rapport in the situation, asking a great question and pausing to allow thinking and listening can really help. Apparently being asked a good open question sends blood and oxygen back to the problem solving bit of our brains. So asking a question gets someone’s brain back in the conversation. You can even do this to yourself. My question that I dredge from somewhere (takes practise – see tip 1) is “Dulcie, what do you really want from this conversation ?”. It’s not perfect, but it comes to mind and does the job.
If you want to know more, David Rock and his SCARF model are brilliant. SCARF stands for Stability, Certainty, Autonomy, Relationships and Fairness. When we feel that one of the things in this SCARF acronym are being threatened, our body produces that fight or flight response. Thus think about what you say today – to others and yourself. Try and find ways to cut to the chase without causing the threat response and making the brain stop working…  
Let me know if knowing this helps.