Loving the wrong person
It feels like you're suddenly falling in a endless hole and the person you love watches you from the cliff because they were careful not to do this kind of mistake, mistake which you did. It's like you're drowning in a ocean of agony and sadness while your loved one breathes perfectly fine because you are the one who was so stupid to love the wrong person.
And when you see them walking by and not giving a sh*t about your feelings at all, it's just like the whole world is crushing down and you know for sure you won't survive. You know you will die because you will be wounded for the rest of your life and no one will be there to heal you. You are waiting for the person you love. You want them to heal you, but you know they will never come. You're just left to bleed until you die. And the fact that you know exactly that this is what's going to happen makes it a thousand times worse.