Things I would tell her
You learn as you get older. You learn things you just can’t have known before, because they require experience to understand—the real hard facts of living. You learn difficult things, such as people will disappoint you. Even the people you love the most. In fact, it is expressly those you love that will disappoint you. But you also learn things of mercy, such as life is a long game of endless forgiveness; of receiving a wound, healing, and then forgetting even the scar for those people you find invaluable, and for those you do not find invaluable, for those you have to let go, you learn you have to forget for yourself.
People like to ask what would you tell your younger self, and I think in addition to all the above I would tell her:
As often as you can, and in as many situations as you can, learn to accept things as they are. Do not spin and scream, trying to change by force of will what only time, and God, can change. Do not resist life. Be sad, be disappointed, be joyful, be grateful, but accept the situation you are in. Pour your energy into trying to change what you can, which, more often than not, is only your outlook, your mindset, on what is out of your control.
Learn to accept things as they are, and you will find in that acceptance not only the peace which passes all understanding, but the space through which the change you seek can enter. I would tell her to loosen her grip. I would tell her that acceptance is alchemy—it transforms swine into pearls. It shifts your gaze from your navel to the sky. It doesn’t make you smaller, but grander, vaster. Then you become expansive enough for the energy of life—the same energy that cracks thunder and fires lightning, the same energy that moves treetops and fills oceans with rivers—to fire through you, to move through you, to fill you. And you learn, as this process happens again and again, that things might not shake out as you thought you wanted them to; they might look different, but they are exactly what you need. You will learn that, living this way to the best of your ability, you will be saved, again and again, not only from excess pain, but from yourself.