Author's Note: This is a short version. I do have plans to republish and extend it at a later time but it popped into my head and hour ago and wouldn't leave so tada.
He needs to stop.
Tom knows that when he stares at it, it shows his greatest desire.
That's why he returns every night.
He comes back to see her, to see the way her eyes shine and her hair defies gravity. He comes back to see the way she smiles at him when she knows something he doesn't and the way she just knows what to say.
He comes back to keep the night terrors at bay, because she is the only one who can.
He knows it isn't real but he comes anyways, to see an illusion, a fantasy, something he can't have, something that is too far out of his reach.
Except that that it shouldn't be because he is the dark lord. He is all powerful, enigmatic, intelligent.
Even so, he knows he can't keep doing this, he knows because any longer and he'll break. He'll fall apart the next time he sees her and she'll know the truth and she'll be disgusted, she'll leave him.
He knows but he can't stop.
He can't let go of her, she is everywhere and she occupies his mind at all times.
He can't help it, he thinks of her in everything he sees; the green of the Slytherin common room reminds him of her eyes, the fire that sits underneath his cauldron is her wild and untamable hair, the storm clouds that gather over Hogwarts is her curse scar and the cello that sits in the corner of his room is the reason he loves her.
He knows, oh god does he know.
He knows he needs to stop before he's too far gone, knows he needs to let go before it becomes all too real, all too hard to ignore, but he can't.
He can't ignore it because he is too far gone, too deep in love with her and it is all too real for him.
Tom knows that when he stares at the mirror of erised, he is only getting his hopes up. He's wishing for something that can never happen, because even if Harry is an intrinsic, crucial part of his life, he will never have her the way he craves to, and that shatters his heart.
Sorry it's so short!