I am sorry that your early years were so very difficult. The first thing that strikes me is the sense of abandonment you experienced from the circumstances surrounding your mother's life choices and mental health. Added to that, was that you were then left to parent your brother...a life role that you were ill equipped to handle. This we call "holes in roles"...meaning that there was a parenting "hole" and you stepped in to that "role".
Then when you met your biological father...a man that by circumstance abandoned you at birth...he abandoned you a second time.
Your mother continues in this theme of abandoning you as she is a medical "mess" and continues to be unavailable to you in a healthy, motherly way.
Now that you are married and a mother...those old wounds are magnified. You cannot imagine how a mother would be so unavailable to their children when you consider your 3 beautiful children! You KNOW they need mothering and fathering. Yet you KNOW you did
not get that...and so it hurts inside. What you may also realize is that your mother and stepfather and biological father gave you such a poor roadmap or directions on how to parent that you may worry whether you can or will be a good mother. This is not likely something that is conscious...but rather unconscious.
As to therapy. My suggestion is that you search out a psychologist who specializes in working with clients with family of origin issues, particularly abandonment. This is the core issue.
I will wait for your reply.