Missing someone doesn't mean it was a bad idea to leave them behind.
My ex gave me a blue eye and bruised ribs when I left him, and guess what, I ended up missing him. Him of all people.
But its all part of the process, and yes, grief is a process, you're grieving the death of your relationship and all the dreams that died with it.
So its ok to miss him, acknowledge it, but distract yourself, and keep keeping on.
If he already has someone else that just means he's even MORE of a heartless jerk than you realized.
It would only be a negative reflection of him, nothing more.
So why worry huh?
Don't unblock him, remind yourself why it ended, please go re-read your birthday thread, that should help.