How do I get through this social event without alcohol?
For most people, the answer to this question is alcohol. But the point isn't actually the alcohol. Most people believe they need alcohol to relax, but this seems to be a conditioned response. If people are told they're drinking alcohol they will behave tipsy. Regardless of whether they are or not.
It's also worth noting that in subcultures where people don't drink, like Mormons in the USA, people are often unexpectedly lively at parties. Because they don't require alcohol to allow themselves to behave that way. Whether you behave more or less friendly or freely is a choice, one you can force yourself to make.
Perhaps the reality is that for most people alcohol is a party placebo. Many assume adulthood requires strict control of one's inhibitions. It's not acceptable to be playful, which we associate with being childish - even though such a state is usually benign and importantly joyful. Basically, alcohol is used by most people as an excuse to be free and have fun in an otherwise tightly regulated social environment.
Most people won't care if you are drinking or not. The point is having an opportunity to socialise. Your seniors are not unhappy about your absence because you don't drink. It's because they want to get to know you better, so you can feel less awkward around each other at work.
If you repeatedly refuse to show they'll feel upset, because that implies you don't care about them. Whether you do or not is besides the point, it's polite to make an effort.
I don't drink. If there's a social event (and alcohol is always part of it) I will go early, stay for a few hours, do the rounds. By which I mean make an effort to say hello to everyone and ask them about non-work things, then leave before it gets too late and they get too drunk. The more people you chat with, the more likely you are to find someone you can really enjoy chatting with.
How are you to behave? Just be friendly. Say hello to as many people as possible and ask them questions. That'll leave a good impression. You might even have a laugh, have a really interesting conversation with a colleague, or bump into an attractive person from another department. Maybe they'll be a shy introvert as well!
I too find many social gatherings draining, and enjoy a considerable amount of alone time. But the bottom line is this: you are at a social event to be social. I have to force myself to be social sometimes, I don't find it natural. But like anything else, you'll get better with practice. It's only for a few hours. The experience is what you make it.