Nervousness is a problem that most people have. Since it's based on your own psychology, you're the only one who can really develop strategies to counter it. The trick is to not let yourself be distracted from properly reading the circles.
I've had this happen to me during marathon maps. Not much you can really do once you're physically affected, try and prevent it happening by being calm.
I had pressure on my playing much when I had any experience in this game. I'm still really bad at keeping myself calm. But as I got little more experience in osu! it helped a little. So that might help you in future. Another thing is to keep your focus on the game and think nothing else. But that's easier said than done.
Oh yeah seems your skills in every way seems to be lot of ahead of me.
If not playcount and empting your mind help keeping you calm and easing your pressure then it's something your might have to find yourself. If your age in your profile is correct (16 years old) then it even might be something you are not aware of yet.
Even though I don't know how people at age 16 nowadays knows their innerself. When I was at that age I didn't gave a fu** of things like knowing myself deepingly. (Don't ask what that meant.)
Anyway I guess there is just so much of things what makes different persons make themself calm (=not pressured) and not sweat in tight situations.
Try and not worry about what's ahead or about losing your Full Clear, step back and remember it's just a game. I used to have this but then I started playing easier maps and became a lot more relaxed about it. Don't think about what happens if you miss but enjoy the game. This is from experience, sorry if it doesn't help!
Getting hyped at the end of a good play is adrenaline. If you keep having that you'll eventually be able to make use of it and do endings especially well. Once the shakyness stops you get better when pumped with adrenaline