Whisper

Nothing; it hurts the most

You can feel the pain by loving someone when they love you back.
*cough cough Cake cough* 😢

You can feel the jealously when someone looks at someone else the same way they looked at you.
*cough cough Cake cough*

You can feel lonely when someone leaves you with no explanation.

But when you feel nothing at all, that's when it hurts the most.