You feel pain, obviously. You feel withdrawal, you feel regret, you feel sad, angry sometimes (both at yourself and that used-to-be special person).
You say things to yourself that you know better than to say.
"What if that was The Last Person I'll ever feel that way about?"
"What if I'll never ever get over this and have to feel this pain forever?"
"What if I had done This Differently and it all could've been Perfect?"
What if, what if.
At some point you develop paranoia about the other person.
You hope that they're going through the same thing that you are. You hope they're feeling your pain -- hopefully more, actually. You want confirmation that you made such an impact in their life that they're just Dying Without You.
How are they coping? Are they talking to their friends? Are they crying themselves to sleep at night? Are they writing about it? Are they going to say Actually, I Want You Back?
Later you feel a little less pain. The nights become easier to bear. You get more done during the day. When your friends ask how you are you'll fake an 'Okay' because you suspect they're a little sick of hearing about it and you really have no new revelations for them either.
And then you think -- wait, is the other person getting over me too? Are they thinking about me less? What the crap.
What if it doesn't hurt for them anymore?
What if they've moved on?
What if they've completely forgotten about me?
What if they've gone off and slept with Someone Else, That Fucking Dickhead? (Never mind that if you did it, it'd be totally justifiable because you were 'emotional,' 'rebounding,' etc)
So what I'm trying to figure out is - why does it matter? Nothing about the actual tangible relationship between you and them has changed, fundamentally. The time you had together was still the time you had together, the emotions you both felt are still Real. How you deal with the aftermath... that's a solo experience. You've parted ways, why does it still feel like it matters?
What's the requisite amount of Pain and Time do they have to go through so that you're 'comfortable' that they've grieved enough?
And furthermore, why do you wish pain upon them to begin with?
It's so hard to justify feeling the way you do sometimes. It's hard to be rational, to step back and say "Okay, what I'm feeling and being paranoid about is Absolutely Batshit Insane." But we do.
I wish I had some sort explanation here. I'm not in the clear just yet, which is maybe why I'm looking for one. I know it gets better. I'm not stupid. It already has, to some extent. But those first moments where things have ended and you're Alone -- why do we make things so difficult for ourselves?