I know using the divine name in ordinary speech has become very common today -- but, yes, I would agree with your girlfriend. The Bible is very clear: "You shall not misuse the name of the Lord your God, for the Lord will not hold anyone guiltless who misuses his name" (Exodus 20:7).
Why is this? In your letter, you say that you "don't mean anything by it" -- but if you stop to think about it, this is exactly what is wrong with it. In other words, your speech shows that you have a very casual attitude toward God and toward Jesus Christ, and that they mean very little to you. Your girlfriend senses this, and I hope you'll be grateful for her spiritual sensitivity.
But should we ignore God, as if He is unworthy of our respect? Or should we treat Jesus Christ as if His death on the cross for us is unimportant? No, of course not. After all, God may be unimportant to you -- but you aren't unimportant to Him! God made you -- and He not only made you, but He loves you and sent His Son into the world to give His life for you. You are that important to Him.
My prayer is that you will face your need of God and open your heart and life to Jesus Christ. Don't be swayed by the crowd, but have the courage to honor Christ in everything -- including your speech.