I can't believe your parents told you, at 16, that they thought you guys would get married one day. Seriously. If I had a 16-yr-old, I'd just be like "enjoy it while it lasts."
Sorry to be harsh, but if this is your first relationship it's going to hurt and be confusing. Sometimes your first 'love' sticks in your mind forever....honestly, for years. A few months isn't going to do it. However, as much as you probably don't want to hear it, you'll meet someone else and fall even harder. The cycle will repeat itself many times in your life.
Understand that at 19, she has the whole world ahead of her after HS and she wants to go out and explore. Doesn't mean she feels nothing for you, but don't count on getting back together.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi