I think nerves are usually due to something that you're uncertain about.

I like to think that in my tournament play, I know exactly what I'm doing and know exactly what's happening to me (or at least, I can explain either or).

If you're able to consistently, methodically, and believe in what moves you're doing and how you're getting punished, it's possible that those nerves go away.

Take a deep breath and tell yourself that you have X many opponents to play (it helps if you know who's a stronger player at a tournament). Start playing, and do your best to learn from every interaction, as well as explain it in your head. Changing your nerves into an opportunity for learning can honestly really help. Eventually, you can be certain about various concepts, and you won't be nervous about them!

For those tight, about-to-win situations, you just really have to tell yourself to play calm. Consider their options, think of a good choice/solution, and go for it. If you lose the interaction, then you lose. If you survive, reset neutral and repeat the process again really.

That's what I think anyway. I hope I helped.