I had a similar experience with my son, losing his sense of worth because of the baby. What helped me the most is the book, Raising Our Children, Raising Ourselves, By Dr. Aldort, and a couple of phone advice sessions with the author. The book is the best I ever read on parenting; so loving, warm, and the ideas resonate with my inner most intuition and it works.

There is a chapter in the book about sibling jealousy, talking about the birth of a baby. I think you will find it extremely useful.