Originally Posted by
Gemlitigate 
I am completely devastated. The intensity of my emotions is actually scaring me. This feels even worse than the break ups and I have no idea why. I left him twice, I know it wasn't a healthy relationship for me, and of course he's going to move on and it's none of my business. But I can't help feeling this overwhelming grief. That he moved on so quickly while I can barely function.
It sounds like your emotions have more to do with the idea of him replacing you with someone else than with losing him specifically. I know my single most terrifying scenario regarding relationships is that the other person will leave me and forget me (which seems highly likely if he finds a new relationship so soon). I´m certain one strong motivation for not leaving my XABF sooner - even when I was having a terrible time with him - was the fear that he would find someone new.
I don´t know where this fear comes from, but I´m sure I´d feel the same if I found out my X has a new gf. But then, the good thing in this case is that we have both experienced what a relationship with these guys is like, that sooner or later this new relationship won´t be anything like the wondrous placid life with Mr. Darcy. It will be plain hell. So try to put yourself in the shoes of this new girl, and just pity her! And thank her in your mind for keeping him away from you.