The Bodhisattva who once gave up. Chenrezig, the Great Bodhisattva of Compassion, also faced disappointment. Here is that story.
Chenrezig once vowed that he will break into a thousand pieces if he drops his vow of enlightening all sentient beings. This is the Bodhisattva vow.
So, after working hard for many eons (1 eon is around 4 billion years), Chenrezig checked, using his divine eye, if the number of sentient beings suffering in Samsara have been reduced.
Yet, he was surprised to know that, despite working to liberate beings for many eons, the number of suffering beings in Samsara have simply increased.
Chenrezig felt disappointed. He dropped his Bodhisattva vow. Thus, he broke into 1000 pieces.
Upon seeing this, his teacher, Amitabha Buddha, saw his state. He told Chenrezig that he will give him a vow that will make him work even harder for all sentient beings.
Chenrezig accepted this vow and he turned into 1000-armed Chenrezig. This is to fulfil his vow of never forgetting to liberate even a single sentient being.