Why does sir Gawain believe he has failed?

Sir Gawain believes he has failed primarily due to his inability to uphold the chivalric ideals of honesty and honor. When he accepts the green girdle from Lady Bertilak, he feels he has compromised his integrity by prioritizing his own survival over his commitment to truthfulness. This internal conflict leads him to perceive his actions as a betrayal of the knightly virtues he is sworn to uphold, resulting in deep shame and a sense of personal failure. Ultimately, Gawain's reflection on his actions highlights the complexities of human morality in the face of fear and temptation.