Beer. If you're too young, have your Dad drink one.
HST, there is an old military saying, "Nobody is dead, nobody is wounded. It's a good day." So, no matter what the results, you will have learned something. And it will be a good day. I often play online with no interest in winning. I'm trying out an idea or practicing a tactic. Which means my rating is crap, but I'm learning.
Nervous energy is because you think of the consequences of losing. Focus on the process of playing the best moves and how you look for them. Think of the process and the result takes care of itself.
Yes, the nervousness is a real thing. But it does go away the more you play. Try to remember that it is just a game, and we’re all amateurs here. Have fun, and try not to worry. Everything’s still fine even if you do lose an occasional chess game. 
You get nervous. You’re human like the rest of us. It’s okay, really it is. I’d say embrace the nervousness and play anyway. You will either win, lose, or draw no matter if you’re nervous or not. Just play and enjoy the journey. You got this!
I don't get nervous when playing online, maybe because there are no consequences. But when I play OTB, I feel a lot of pressure and anxiety.
My pre-game warmup before I start my 30 minute bullet escapade consists of eating a carrot, taking a walk outside, and punching my pillow after I lose 50 rating points.
In all seriousness, just enjoy the game! Your rating is just fragments of computer code on a digital screen, and your losses can be beneficial for future improvement. Take your losses as a learning opportunity. I know it may be hard to see it in this perspective at first, (in fact, I still struggle with this as well), but once we identify why we are playing the game in the first place we can begin to enjoy the game regardless of the outcome.
Sometimes, before I play an online game, I get nervous. I am getting better at controlling said nervousness (I actually had a period where I wouldn't play chess online at all in fear of a loss) but I just wonder whether there are ways to help with this.