It is knowing that someone lives with a part of you that you cannot ever reclaim, no matter how close you get to them. There is no way to truly own another person, but you try anyway because you want--you need--to make sure that that most vulnerable, raw part of yourself they have somehow stolen is kept safe. You do everything in your power to get It back from them and in so trying end up somehow giving up even more of yourself.
You learn immediately to adore them for all their wonders almost as much as you realize you must hate yourself for being so unworthy of all that they are, and then you hate them for making you hate yourself, and then you end up walking around in the middle of the night like a crazy person, trying to put some of the emptiness and darkness inside your chest to crowd the other things out.
True love is when you look up stupid dating advice, and ask all your friends for guidance, and end up setting self-help books on fire because you realize that for all the pondering and all the questions, in the end, nobody really knows a single stupid thing about love. Everyone in the history of the world thinks they know something about love, and if every single one of them told you to give it up, you'd have to tell them that you can't.
It is the great, rushing human problem.
mom-"True love is very rare."
girl-"I know mom, I know, that is why I shall cherish him forever. I'll cherish him past death, love him for who he is."