yeah staying calm is super important. if you make gut reactions to attacks then you will likely blunder. if you stay calm and make informed decisions you will have a better chance at defending it.
This chess video really helped me...
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Funny enough, this video is about playing at a ~300 ELO level of a larger series. Played by a NM. But the way he talked about the concepts, things to look for, the ideas you want to achieve in a very basic manner ... it has really changed my game for the better. I went from full tilting, to winning games at the ~1200 level now with (relative) ease, at least compared to how I was playing before.
I'd highly recommend giving the video below a full watch if you're struggling in your games. I think it will help anybody in the sub-1500 category.
The biggest thing for me was noting his demeanor. He remained calm, even when his opponent was on the offensive. Of course, for an NM against a 300 ELO that's easy to do. But for me, I was always quick to react and worried I was missing something or in some sort of trouble. Watching this video helped me understand the best thing I can do is take my time and relax, and realize it is probably my opponent who is the one making a mistake, I just need to be patient and find it!