No amount of metaphor will fix the damage that unrequited love does to the teenage heart. One day, you will find someone, you will look at them like they are magic, and they will look at you like you are not. Just because they do not love you back does not make them a galaxy or an ocean or a night sky or something poisonous inside of your veins—it simply means that they do not love you back. You will want to compare them to something so vast and infinite that you will not have to claim the responsibility of cleaning up their mess. You will do it anyway. You will still be magic without them.
You might cry for a day, or a week, or several months, and it will feel like your world is sinking in some kind of apocalyptic flood where you have forgotten how to swim. It might feel like drowning. It might feel a little like being shot. You will blame them and yourself in equal parts; remember that you are both still young and every problem fades with enough time and chocolate.
Remember that it is not your job to fix anyone. You will want to cut into them, change their minds, mold them into someone you can curl comfortably into the curves of. Don't. If they cannot change without you, they will never change with you. They may not know how to yet. They may not want to face the parts of themselves that you remind them of. Don't blame yourself for their fear.
There is no pain Ben and Jerry's ice cream can't fix.
Let yourself wallow, just a little bit. No one will be able to tell you to "get over it" convincingly enough to actually stitch you back together, so let yourself fall apart. The energy it takes to force together a star that wants to become a supernova is dangerous when held captive for too long. When the sun comes up again you'll remember that the galaxy actually didn't fall down around you while you were looking the other way.
Baring too much of your soul will always be better than hiding it inside of your ribcage. Don't just wear your heart on your sleeve, wear it in the soles of your shoes and the breast pocket of your favorite coat, too. There will never be a moment where there is too little of you left to give away, so be generous with yourself.
Let them teach you about yourself, your heart, others, and the world. Let them hurt you and support you and guide you through the mistakes, even if they don't know how those things will change you. Let them be, and don't despise them for being something you didn't think they were. Forgive.
Love yourself in the end. That will be the only happy ending you ever need.