Maybe there are other positive qualities that are hard to let go of in your ex, and that hurts to grieve the loss, but that doesn't mean you should get back together with him. Boring and little physical attraction are two major deterrents to being able to be happy within the relationship. Endings, regardless, require a grieving period and it sounds like you're still grieving. Perhaps don't get serious with the new one and allow yourself more time to grieve the ending of the last. We all need time to grieve, and it can take time to allow the sadness to move through us.