I think the hardest thing about ending a longterm relationship is knowing that you will be losing the person who knows everything about you. When times are hard, or you feel sad, or even elated, you want to share those moments with somebody who truly understands you and appreciates you.
Fizzy is right though. I'm sure there is a valid reason why you both broke up. You have a lovely partner now so concentrate on building a close bond with them. It does take time but it'll be worth it in the long run. x