Should I leave my fiancé who doesn't pray and associates with bad company, knowing that I love him and fear that I won't forget him, or should I stay with him and try hard to bring him closer to God, even though I have started to compromise on some matters that displease God because of him?

Engagement is merely a promise of marriage, which does not permit the suitor anything except to look at the fiancée. She should be treated like any other non-mahram (definition) woman; thus, private seclusion (khalwa (definition)) with her, touching her, or speaking with her without necessity is impermissible. Sincere repentance is required for any transgressions during the engagement period. The woman's guardian must choose a husband who is righteous in his faith and character, because a man is the protector and maintainer of a woman. Your fiancé does not pray, which is a serious matter; for abandoning prayer may be considered disbelief by some scholars, and it is not permissible for a Muslim woman to marry a disbeliever. Distance yourself from him if you seek Allah's pleasure, and seek His help in overcoming your attachment to him. As for your lack of commitment, it requires striving against the self, patience, supplication, remembering the Hereafter, accompanying righteous women, and attending gatherings of knowledge and remembrance.