Well, of course you will have a "terrible time" if that's the mindset you go with!
Why not focus on the wedding and what it is about, instead of your own needs? Why not use this as an opportunity to develop your social skills? If you go through life refusing to do anything without a boyfriend in tow, you're going to miss out on a lot of life! You won't be sitting by yourself, you'll be at a table with other people...probably others who are there solo, as brides usually keep this in mind....and there will be people to chat with and even have fun with, if you go with an open mind.
I am mystified why you (or anyone else) thinks they are entitled to bring a date to a wedding, regardless of the bride's wishes. And I don't think the living together argument is compelling. Nowadays, there are plenty of people who routinely move in with their boyfriend/girlfriend du jour, so living together does not always signify the seriousness of a relationship, and even if it did, it is still not your day, it's the bride's day.
It seems that it is more important to you that the bride recognize your current relationship status, than for you to recognize this important day in her life. Part of being an adult is doing things on your own, and it's really a shame for your friend that you can't go solo for a few short hours out of your life in order to celebrate her marriage.