What does your heart tell ya to do?
I am with someone and now a second girl has messaged me saying she wants to go out with me. What do I do? I'm not a cruel person, I don't really want to upset anyone.
I've had this happen to me (where I was the girl who liked a guy who's going out with someone else) and the best thing was that he told me straight up that he was going out with someone else. He also said it nicely too, along the lines of "You are a good person and I admire/respect you. But I'm sorry, I'm currently in a relationship with someone else right now and I want to see how things work out with her." That was it.
And after that he didn't avoid me or anything like that but he was more careful with what he said to me (although after he rejected me I didn't really care anymore although I still appreciated that he was so nice about it). Maybe you could try taking that approach?
Go with him. Don't break her heart.