Have you ever planned out your entire day only to find it all thrown off by one event? It doesn't even have to be something huge... The printer/WiFi stops working and you can't print your assignment, the bus or train doesn't come on time, there's traffic where you wouldn't expect there to be, and so so much else. It might even be something fun like an impromptu family get-together or having dinner out with friends, but it still stresses you out. After all, you could have been finishing work and getting other "more important" things done.
First, it's important to understand where that stress comes from. A significant portion of it comes from the fact that you're really committed to your responsibilities. Put simply, nerves mean you care.
But what's the point of stressing over something that was never in your control? You do what you can and after that, just try your best.
I want to let you know, though, that it will always, always work out.
Those moments happen for the sole reason that the universe is trying to take care of you and tell you that it is more than okay to let loose.
Honestly, it's something that we should do once in a while. And when we don't do that for ourselves, it's done for us, for our own benefit. I feel like we tend to lose sight of this truth very quickly just because of the way our lives are structured: around work and deadlines and whatever the next thing may be. It's all about living from one exam to the next, one project to the next, etc.
And, for the practical side of our brains, the one that can't bear to do something without seeing the black-and-white logic of it, know that spontaneity develops that part of you that few other things can. It makes you well-rounded for the sake of your own good, for your own long-term gain.
Invest in yourself.