Is it possible your energy is directed towards that problem in social situations? The exact same thing happens to me, and I've realized that's what often happens. Or perhaps just anxiety in general. That can detract from others uses of your mind, and, therefore, once you're removed from the source of anxiety, you think of everything you wanted to say then. But... It's likely you already know that.

I've been trying to develop a solution to this for a while, although the methods may not work... But ideas are ideas.

One observation is that when I stop hearing "other people saying smart things" and replace it with "an interesting exchange of ideas", my anxiety decreases and (natural!) contribution increases. Normally, the harder I try the worse I do (this correlation is quite strong!).

Assuming you experience at least relatively similar feelings, perhaps these methods will help you, as well. Or maybe you'll find something else. Are there any topics that inspire you? Try talking about those things and maybe experiment a bit, "flow" will be achieved most easily and you can apply what works in those discussions to more challenging topics.

Best wishes!