My no 1 & 2 are quite far apart in age with a 6 yr gap. But I remembered when my no 1 still wanted me to carry her around while pregnant.
Firstly, It's important to teach your child that you are pregnant. Involve no 1 in your pregnancy. e.g. Dr check ups and stories about pregnancy as well as becoming a new sibling.
Secondly, you need to learn to let go when she cries needing you. Let your husband or family member coax her through her rants. Mothers are after all, a child's safety blanket. Since you are going to have another baby, it's even more vital to trust your family to help. Go to another room or go out away from your child to allow her to trust the other members more.
By doing this, you are also teaching her trust as well as independence.