How do you deal with failure?

I just realized a “once in a lifetime opportunity” has just passed me by, because I did not pay enough attention over the past year or so… thinking nothing could change (so rapidly) in this respect… and yet it did, and the chance is now lost. I am struggling to “go with the flow”, and accept what I no longer have any influence over. The realization that it’s “game over” though, that I could have easily prevented a negative outcome if only I paid slightly more attention… it’s actually causing me some mild chest pain now, I feel dizzy, somewhat sick to the stomach… need to lie down. Talking about remaining calm in situations like this one and actually doing it = 2 very different things as I am discovering (yet again).

Summary: how do you remain grounded, when you recieve news that absolutely shatters you, knowing you could have done something to prevent it from hapenning (or so it seems)? How do you regain peace of mind after something like this?

Cheers!