I know it's hard to believe but you are actually in a better place than your ex who has "moved on". That's because after such a long relationship, no one can be ready to move on as quickly as he did. All he did was pack up all his mental baggage and move it from you to her. He still has it and is unpacking it on her now. She'll find out soon enough. He has not dealt with it.
You, on the other hand, have a conscious mind and heart that is telling you that it is too soon to move on and date again. Your conscious is protecting you from jumping into another realtionship so fast. You have not had the time to heal and re-evaluate your needs and become independant again.
The way I see it, YOU are dealing with what needs to be dealt with NOW, the pain, the processing, the grief. Your mind is telling you that it hurts and so you are very wisely listening and not jumping into another reationship yet.
Your ex on the other hand, is NOT dealing with anything at all. He's just in denial and thinks he can move on and all this baggage and his issues will somehow magically dissappear in a new realtionship. They won't. They will all surface with HER.
So before you wonder why he is so fortunate to be able to move on from you so easily, don't. Because at some point he will have to feel the same pain from healing that you are. Even if he goes through seven more realtionships that fail first thinking he can run from his issues, eventually he will face the same music you are facing now. Unless he plans on never having a healthy realtionship with anyone and ,well, that's his problem, right?
So as bad as it sucks, at least you can know that the pain you feel is normal and healthy and necessary to move on and heal. You're doing it, even if it feels like it is taking forever. Your relationship was a long one, so don't beat yourself up over how long it takes you to "get over" it. It will happen, with each passing day. It will happen!!! And guess what....your ex will still be out there living in denial and most likely unleashing all his issues onto some other poor unsuspecting woman. Be happy that is no longer YOU!