Under confident when I dont know something

So my under confidence is mostly in work environment. When I dont know something and I get cornered and I dont appear very confident and for a moment I go blank with what I have to say. The general approach is to learn about it and be prepared but honestly that has helped me little because the interaction with stakeholders are so unplanned that you might be called for a discussion and you are not prepared by then.

Is there anything else I can do which can help me in

maintain my confidence in such situations and I or my words don't sound under confident.

If someone has faced such situation would love to hear how do you deal with such situation.