How do I learn to be alone?

I've already made a post about this, but even now it still hurts me.

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I, F17, lost two of my close friends due to some stupid issues and geniunely spineless behaviour. Just so much disrespect and lies that I cannot believe that I used to trust these people with every inch of my being. I'm not used to being this alone, since one of them was apart of my life for half a decade—and they threw our friendship away like it was nothing, moving on to the next best thing in an instant. I know that the relationship was getting unhealthy, I know I should protect my peace, but my heart cannot believe with what my mind already knows.

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I'm at peace knowing I'll never see them again, but I have one more year of highschool left, and I don't know how to deal with seeing their faces everyday for the next 9 months. I'm so tired. I feel like I'm constantly carrying around this heavy ache everyday, and I'm continuously living in my head by repeating it over and over again. What do I do? I don't want to rely on people like that ever again.