It begins when you can’t stop bringing them up in casual conversation, even to people who don’t know them or don’t know you.
You know when they are happy or sad or lonely, even when they won’t own up to their feelings, because you somehow just sense it.
You find yourself accidentally talking about the future, even when you don’t mean to, and it just happens to slide into conversation.
You learn to differentiate between love, lust and attachment.
Realize that relationships aren’t supposed to make you feel good as much as they are supposed to teach you something, and to help you grow. Allow someone to be part of your story and not the tragic, final scene. Take from it what you need to learn.
Let yourself be sad. Resisting it makes it worse than it is. Give yourself time to mourn and grieve the loss of someone who was a big part of your life. Then let them go, and love yourself enough to let yourself go too. As in, let yourself walk away from them in the metaphorical sense. Nobody else will give you permission.
Embrace that you may never get over them, but let them be a part of your story anyway.