I work with high-level English learners speaking in business, work and day-to-day life. This is a very common problem. There could be a couple of reasons why it happens. But the biggest are:

1. You haven't learnt the language well and
2. You've practised in comfortable situations.
3. A third problem is that people simply don't know what to say (i.e. their English is fine, but they don't have anything to say).

Let's break this down. We tend to think of fluency as being one thing, but that’s not true. Fluency is actually FIVE things. They’re all happening in your brain together, and that creates the illusion of fluency. The first of these things is what we call "encoding”. You haven't learnt the language well Have you learnt the English you need, in a way that properly puts it into long-term memory where it can be found and used? If you're memorising random words from lists, that won't work. You need to learn them on a deeper level, together whith the phrases, expressions and chunks they are commonly used in. There's a detailed (and free) training here which will get you started with this kind of "deep" learning. You've practised only in comfortable situations Another one of the five elements is what we call 'cognitive load’. And this is the most common reason your mind might “go blank”. It’s like your computer. If you've got a hundred programmes running at once, they’re all going to struggle to run smoothly. Eventually, you get the blue-screen of death, and your computer shuts down. But if you close everything down and just let one program run, it’ll do so efficiently. Or, to extend the metaphor, "fluently”. What this means is, if you’re not confident and you panic and get flustered, you’ll struggle to speak because your mind will be taken over by other things – the panic and the fear and the fight-or-flight response that comes with it. There isn’t enough “RAM" left for speaking well. A lot of the time this is caused by practise in easy, comfortable situations (like a conversation teacher or partner), but not enough is scary, real world situations (see here for an article and video I wrote about this). The solution is to free up your brain as much as possible. Learn English in the right way (study this free training), know WHAT to say and HOW to say it in conversation. Then get good (and get over your fear) by using English in scary in real-world situations. Hope that helps. Best, Julian Northbrook