You indeed were not given the correct information in the first place. The bad news is, the higher rate is correct. You have to be married to your girlfriend or you have to be an owner of her vehicle in order to get the multi-car discount.
A State Farm Agent does not have the ability to say "my bad" and give you the discount anyway. It is against the law. They have to rate you properly. It is also illegal for the Agent to pay the extra portion of your premium.
I have State Farm Insurance, and I will tell you that they are always rated right near the top when it comes to insurance companies. Sometimes human error creates a bad situation. I don't believe that Cilia did that intentionally.
Sorry that happened to you.