I'm mad the girl I have been talking to spent valentine's day with another guy
She must like that guy instead of you.
If they care about each other, they will tell me the reason.
I understand you are feeling sad about it, but they do not have to tell you. Their relationship is private and you need to respect that.
We were supposed to be together.
You may feel that way, but she does not. Try to find a different valentine, there are many more people in the world.