Good luck.
I try to sit on my own for a while before any performance. Just close your eyes and play your peices in your head. This works for me.
You say you have only been playing for four years? you are doing very well if you are 'performing' infront of people semi regularly and playing things like the schumman concerto.Most people after four years still don't know all their scales! Congratulations. You must work very hard.
Wishing you the best! Have a wonderful time playing. If you're having trouble breathing--which I completely understand--try holding one nostril closed and take several LONG, SLOW breaths, all the way to full, with the open side. This should take a few minutes, really stretch it out. Then switch sides. This is calming because it helps you to take in more air, thus more oxygen, which you will want a lot of today.
You can also think about all the nice people who are coming to hear you play, people who are looking for beauty and a little escape. You can give them that, which is a privilege. Also, don't be too afraid about being afraid, if you know what I mean. You can be scared and still play very well... so if you start feeling a little wild, don't think that it will ruin your playing, because it won't at all.
As far as playing on concert days, I like to play through my pieces once early in the day, very slowly on fast pieces, and calmly, like each one is a long lullaby. It lets me think through my pieces and their structures in a calm way, and it dissipates some excess energy from my fingers without being tiring. I don't play big, or play at tempo, or play too close to the concert--all those things are too stimulating for me. I do a 3-minute octave and scale warmup a half-hour before the concert, and that's all the real (energized) playing I do the whole day. I also like to see a movie, but only if it's trite and funny, nothing scary or too sad.
its good that you have taken the day off work, but you still need to occupy yourself with something, but not tire yourself out. but you shouldnt just laze around. go for a small run or some long walks, work your mind on other thinks but not work.