It is true and I have never thought about it. I have always judged Mary and have not thought about her situation. She lost her mother when she was very young, maybe she even doesn't remember her. As you all have said her father and older sister don't care about her and she receive some little attention from Lady Russel. She had to leave for school alone and lived a long time I guess out of the house. So the poor girl certainly felt alone with no family for a while until she got married. This circumstances can explain her lack of confidence.