In the story, Mariam is indeed a figure of sacrifice. Here is one quote that demonstrates this:

She could feel his hand working at his belt, at the drawstring of her trousers. Her own hands clenched the sheets in fistfuls. He rolled on top of her, wriggled and shifted, and she let out a whimper. Mariam closed her eyes, gritted her teeth. The pain was sudden and astonishing. Her eyes sprang open. She sucked air through her teeth and bit on the knuckle of her thumb.

Here, Mariam's meek submission to Rasheed leads to physical injury. Essentially, Mariam sacrifices her feelings for Rasheed's selfish pleasure. She does this not because of a misguided desire to please her husband but because society demands her obedience to her husband. For a woman such as Mariam, there is no recourse to justice.

For her, all these months later, their coupling was still an exercise in tolerating pain. His appetite, on the other hand, was fierce, sometimes bordering on the violent.

The above quote demonstrates that pain is a constant companion for Mariam in the beginning of her marriage. She sacrifices her physical welfare for Rasheed because of her fear, sense of responsibility, and low self-esteem. Mariam believes that her plain looks consign her to a life of servitude to an abusive husband. Even after she finds Rasheed's pornographic magazines, Mariam tries to rationalize her husband's brutal treatment:

He was a man. All those years without a woman. Could she fault him for being the way God had created him?

A few more quotes that show how Mariam sacrifices herself are:

I won't have the two of you living on the run, like fugitives. What will happen to your children if you're caught? . . .

Who will take care of them then? The Taliban? Think like a mother, Laila jo. Think like a mother. I am. . . .

For me, it ends here. There's nothing more I want. Everything I'd ever wished for as a little girl you've already given me. You and your children have made me so very happy. It's all right, Laila jo.

Here, Mariam sacrifices herself so that Laila and her children can escape to safety. Mariam understands that two women on the run will alert the Taliban. So, she chooses to stay behind to take the blame for Rasheed's death. Mariam spends ten days in prison before she is executed. Her last thoughts are of Laila, the children, and God.

Quotes that show how Mariam sacrifices herself in A Thousand Splendid Suns display the extent to social practice challenges female identity.  

One example of Mariam sacrificing herself can be seen in her marriage to Jalil. She recognizes that in Afghanistan, marriage is synonymous with reducing a woman's identity.  Over the course of her marriage, Mariam understands how this is the case in her own:

It’s not so much what he says, the blatant lies, the contrived empathy, or even the fact that he has not raised a hand to her, Mariam, since he had dug the girl out from under those bricks. It is the staged delivery. Like a performance. An attempt on his part, both sly and pathetic, to impress. To charm. And suddenly, Mariam knows that her suspicions are right. She understands with a dread that is a blinding whack to the side of her head that what she is witnessing is nothing less than a courtship.

Mariam knows that her husband is wooing another woman, thereby reducing her own status.  However, there is nothing she can do about it.  She understands that her life is going to be one of sacrifice.  The quote also shows that Mariam must accept the fact that Jalil will never accept her as a soul mate. She must sacrifice any hope of finding happiness as a wife, a "blinding whack" of reality. 

Mariam sacrifices herself when she kills Jalil.  She does so to save Laila's life. Doing so means that she must accept the punishment of death that Afghan society sanctions:

After Mariam was punished with a sentence of death, she was led out to sign a document while the Taliban watched. “She wrote out her name - the neem, the reh, the yah, and the neem - remembering the last time she signed her name to a document, twenty-seven years before, at Jalil’s table, beneath the watchful gaze of another mullah. 

The signing of her own death warrant represents the sacrifice that was her life. Mariam sacrificed herself the last time she signed a legal document, her marriage certificate.  She now literally sacrifices herself in signing the document that guarantees her death. 

In her final moments of life, Mariam reflects about the sacrifice she made when she says,  “This is a legitimate end to a life of illegitimate beginnings.”  Mariam understands that in order to change the condition that faces women in Afghanistan, sacrifice must be made. She takes an active step towards this in her actions.  She wishes to legitimize something that society had deemed opposite.  When Mariam "does as she is told" for the last time, it is clear that she understands what it means to sacrifice.