In Khaled Hosseni's A Thousand Splendid Suns, which character seems to be more insecure and why?
If you are discussing just Mariam and Laila in regards to insecurity, then Mariam demonstrates more issues with low self-worth than does Laila. The difference between the two women besides their appearances and age is that Mariam has truly never known love. Her mother calls her a bastard. Her father manipulates and abandons her, and her husband cares only about what he can get from her. When a person experiences those types of relationships only, she is bound to question her own worth as a human being.
In contrast, while Lalia is broken once she becomes a second wife, she has at least known what is like to have parents who want what is best for her and who cherish her. She also experienced the loveof Tariq who chose her above all others, and later her child shows her unconditional love. So, while her marriage and Mariam'sinitial treatment of her affect her self-esteem, they are not enough to erase the knowledge that she was once loved and cherished.
Mariam is the more insecure character. To begin with, she was an illegitimate child, and thus her social standing is very low. She is well aware of her standing in the community and has her poor self-esteem reflected back at her by other people who treat her as less than others.
Her husband is abusive and tries to keep her down, inhibiting any self-growth or personal improvement on her part.
That all changes when Mariam links up with Laila. Laila is an inspiration and helps Mariam get past some (not all) of her insecurities.