Don't feel dumb. Appear unaffected. Don't accuse her of anything. Wish them good luck.

The way people will tell this story and that you tell it yourself may have a lasting impact on your self image. Reality is fuzzy and therefore we make sense of it through stories. However, there is not just one true story, so pick the one that suits you and that is in line with (verifiable) facts. Often, you just have to reframe the situation: E.g., YOU were the one who had her even though she was not even in love with you. So YOU used her in a way as well.

Don't show interest in her. This would contradict your story and would give her power over you and her ex, who might have just returned to her because of jealousy and might leave her again once it is gone.

Emotions are widely involuntary and subconscious, but they are fed by our believes (the shark in jaws isn't real, but we believe it anyway). So tell yourself your story and the pain will go. As a rule of thumb: the person who shows emotions is often seen as the the looser or the one who is pitiable. So keep calm and appear unaffected and soon that story will become your reality.