I doubt there is any correlation between a person living with a boyfriend/girlfriend outside of marriage and their ultimate divorce rate when they do get married.
Most of my college teammates lived with a serious girlfriend in their 20s. Some of them stayed together, some did not. Most of them, including the ones who broke up with past live in partners, are now married and apparently stable. Divorce is actua)y fairly rare among college graduates, partly because they wait longer to get married.
As a married man now, I look back on my two live in relationships with a lot of fondness. Marriage really is about kids and money. A live in relationship is about you and your partner.
To the OP, Don't pretend to be married or treat it as a trial run.Enjoy each others company and don't be petty. You will either be compatible or you wont. No amount of social negotiation is going to change how you react to day to day interactions.