Letting go is one of the hardest thing to do as a human. This is because when you are in love with someone, you are emotionally attached to the person and it is not easy to let go of an emotion you are use to.
Letting go comes with a lot of changes in your life, you have to avoid certain things like places you and your ex love to go. It is quiet difficult to adapt to these changes, that's why it is letting go seem to be very hard.
When you love someone, you are emotionally attached to him or her. You get addicted to their presence. Their daily texts, habits and activities become part of your life. You start liking the routine that you build with your lover. Their scent becomes yours and their voice becomes a regular hymn.
Letting go of a person means changing all these little things. You need to incorporate hundreds of changes. Many of your habits are associated with your spouse and they change too. Your entire day transforms suddenly and so does your life. It is difficult to accept this change. This is why it is so hard to move on.