She is a high school student. I think, as a parent, you need to be supportive to her IF she asks. Be a good listener, not an advice giver. Let her talk this out but only if she initiates.
HS kids break up with significant others all the time. Heck…so do college students.
Please, let this go. It sounds from what you wrote that you were completely wrapped up in this relationship…maybe more so than the parties in the relationship.
As hard as you view this…just move on. The relationship has ended at least for now. Just move on.