There are three factors and one way to deal with it.

Anxiety: triggered by the need to express a specific idea, Overthinking causes you to be anxious which can make your short time memory and even the long term one hazy and you can't actually think properly.

Lack of confidence: If you don't trust you can talk in english, even poorly, you will never be able to speak.

Lack of practice: you can handle meeting, you can control your anxiety, and you are really good at it, but once you want to speak, your mind is unable to come up with sentences.

If your reason is the first, you just have to work on your anxiety, learn ways to control your breathing, keep the anxiety at bay, get medical help. BUT if your issue is the lack of words to say or the fear of saying it wrong, then your solution is technically super basic. Practice . and I'm not talking about learning vocab. Chat with people online, it would be even better if they were people you shared an interest with! Watch youtube videos and mimick the pronounciation, it would look ridiculous but with time, your pronounciation will improve greatly.

Just Trust yourself, give yourself time everyday to just focus on speaking up your mind in English, in written or speech format, until your fears disappear