What is meant by Sergeant Marx's final statement that he has accepted his fate? What is his fate?

Like the soldiers who have to accept their assignments and their respective futures, so too does Sergeant Nathan Marx have to accept his position in the military.
Marx spends "The Defender of the Faith" reluctantly doing special favors to help Private Sheldon Grossbart observe his faith. For example, he makes an announcement to the people he is training that they are allowed to leave on Fridays to attend Jewish religious services after Grossbart complains that people have discriminated against him for wanting to do so.
The story continues with Grossbart asking for more favors and Marx reluctantly granting them. Once Marx realizes that Grossbart is tricking him to get special treatment, Marx turns on him and warns Grossbart to stay away from him. However, he finds out that Grossbart is receiving special treatment from someone else and has been given a safe, desirable assignment in the continental United States instead of being shipped overseas with the other recruits.
In response, Marx makes a call and gets the assignment changed so that Grossbart has to go overseas with the other young soldiers. Grossbart weeps as Marx leaves the room after affirming that Grossbart will have to go. He sees the young men "trying as best they could to accept their fate." Continuing, Marx thinks, "Behind me, Grossbart swallowed hard, accepting his. And then, resisting with all my will an impulse to turn and seek pardon for my vindictiveness, I accepted my own." 
Marx means that he has accepted his position as the trainer of young men who are learning to be soldiers. He sees now that he has to treat them all equally and prepare them to face combat. Marx himself has only just returned from fighting in World War II, and he is no longer one of the ordinary troops. Rather, he is in a leadership position, and he has to represent all the men equally. As Captain Barrett tells him in the beginning, "Consequently, Sergeant, nobody gets special treatment here, for the good or the bad. All a man's got to do is prove himself." 
Marx also has to accept what he has done. Part of him wants to beg for forgiveness because he made the decision to send Grossbart with the others. Though Grossbart initially escaped from overseas deployment due to what was likely another favor, it had nothing to do with Marx. He could have let the boy stay closer to home. Instead, he was angry about being fooled and opted to step in and use his influence to have Grossbart sent to the Pacific. As a result, Marx has to accept that he is the kind of person who chose to do that.