As a middleaged-to-older fella who has been playing bass for a decent spell, I find your anxieties refreshing and inspiring. As terrifying as those fears may be, they also speak volumes about how much music means to you, and how excited you are to execute the performance you've been working so hard to make great.
In my humble opinion "butterflies" are not necessarily a bad thing. Some of my favorite experiences performing music have been shared with those pesky rascals. And sure, there have been those nights where they have gotten the better of me and tripped me up here or there. But in general I have always just tried to make peace with them.
It might sound corny but my advice is to develop a relationship with those "butterflies", or "nerves", or whatever you want to call them. Consider them a blessing and try to make a deal with them before the show. Something like:
"Listen nerves, I've heard you out and I understand your concerns. As you are well aware, I share many of those same concerns which is why I practiced all these parts so many times. But now it's showtime. I can promise you I will do my best, but even the legends make mistakes sometimes and I might even make a few of my own tonight. And if I do I can't have you freaking out and making those mistakes even more obvious. Look, I appreciate your excitement and I'd love to share the stage with you. But I swear, if you freak out and make a scene i will have no choice but to confiscate your backstage pass and have security show you to the nearest exit... Do we have a deal?"
Or whatever works for you. Everybody's different. And for me that nervous energy often seems essential to attaining an inspired performance.