I think why someone difficult to move on from ex:

 1. May have been too long in living relationship and very dear.
2. Memories that once experienced with him could leave a distinct impression and make us hard to move on.
3. Still always want to know anything related to the former. What he did, what his current state, has a boyfriend or not, whether ex still loves us, and other things that are not far away from all of it. With this alone, it can be hard to move on cause we 4.Hope he can be together again with us and still love us.
5. Think that he or she is the best ex of other best.
6. Not confident, after breaking up is hard to accept something new, because it was always thought could not get someone as good as him.7. Always felt there was ashortage on yourself or on another person who tried to approach us.
8. Because the ex is able to provide care, comfort and smile.
9. Because I still remember the story of the past, it could not make myself into a better.
10. Perfect physical or handsome makes us hard to forget.
11. Many of the lessons that we get together when ex.
12. Still keep giving stuff away, so that her gift will continue to remind us of with ex.

13. Used to have a boyfriend or girlfriend who is always there when we needed. so afraid to start life as a girl or a boy who singles.
14. Sometimes women are not satisfied with the explanation of the ex when their relationship ends. a wedge issue that makes many women still think of her former lover. More and find out what the cause is, the more we are obsessed with the ex.
15. May be thought of the same favorite song with him when ex first, then because of that song, we suddenly flashbacks all the wonderful memories of the time with the ex, from the time of the streets and other.
16.Because the ex shadows linger in our brains, To the extent that we are willing to reach him. We did it just to see how he was doing.
17. Less activity, also including its causes hard to move on because if we are less busy as there is no work, or stunned silence. This often makes us remember each former, so when it shows up thinking he and his late challenge failed to move on.
18. Not bear, still can not leave him alone or can not forget the memories even still he could not relate to that one cause of failed move on.

19. Maybe one of them is still often visit a place full of memories with him. Memories of the ex so entrenched in our minds, so we find it hard to move on from it. so we are willing to take the time to visit places full of memories with the ex. All that we do to cure her homesickness.

20. Every time I saw someone who looked like him, we suddenly imagined him, and instantly all the memories with exposed clearly in front of us. If we experience it, it means that we are still hard to move on from the ex.

it is my opinion about the difficulty a person move on from the ex, thank you :)