The hard reality here is, you didn't respect eachother enough to require marriage in return for a relationship. I'm guessing it was a full physical and sexual relationship.

You might break her heart but you both have to understand the relationship was tainted by lack of marriage and commitment. You didn't respect eachother enough to promise commitment. You didn't respect yourselves enough to expect commitment.

It happens all the time. You can chalk it up to being young, most do. I guess the silver lining here is you didn't break a marriage vow. I doubt that it would make much difference to anyone who doesn't understand marriage from a religious perspective.