My girlfriend cheated on me?

[Nature Lover]

Lucky you, since it happened when you are still single.
If she did it once, she will do it again, a sign she has no respect for your devoted love.
You need to break up the relationship, which is going nowhere. It will hurt (you) because of your devotion. However the crush will pass and become a memory.

You can not change a person, especially her type. If you try to, you will be hurt even more. Perhaps it was her way of saying the relationship was over, especially when advertised via Facebook (an idiotic action).
Learn from the experience, and go on looking for a girl of a higher standard whom you can trust.
Now you understand what all the songs about "Breaking my heart" mean (both males and females, lol).
You will survive, and some day soon will be glad she did that for your sake.