It does take a long time to get over what you are going through, but you do get there. It's two steps forward, one step back, I found. And yes, sometimes they do want to come back, but quite a few friends have discovered later on that it has been because the other woman had got cold feet.
There are all those stages that you are warned that you will go through, Denial, Anger, Bargaining, Depression and Acceptance. Yes, it is just like a bereavement. the person you knew is now dead to you and a cold and occasionally cruel person who suddenly seems like a stranger has taken their place. If you really do still want him back, and if his relationship isn't going to work, it will probably be a year before the cracks start to show for them. Until then they are still in their honeymoon stage.