Even the person who wanted to end a relationship and did leave will have to mourn the loss of that relationship. The "trigger" is different for everyone. For the person who did the leaving, that can occur when the person they left meets someone new. Now it seems your Ex has "replaced" you or is attractive to someone else.
I initiated my seperation, and I started dating someone 2 months later. But still, when a month after that my ex filed for divorce it hit me like a ton of bricks. The reality that I was getting divorced. Ofcourse I knew he had every right to file- I was seeing someone, but what I didn't know was that I had a right to grieve. Even if you wanted the seperation, you will still need to grieve the loss of your marraige. It happens when there is a finality. Once you know there is no turning back, and you need to grieve to move forward. Counseling helps alot.
Good luck, it's gonna be better.
Welcome to the board, Patty.