How does an 18-year-old girl rid herself of feelings of vanity and self-preference over others when she performs good deeds, and she fears that this feeling is among her sins?

The servant journeys to Allah between fear and hope: fearful of his sins, yet hopeful for Allah's grace and mercy. He perceives his deeds as few and yearns for Allah's forgiveness and acceptance. He recognizes that all favor in obedience, repentance, and righteous deeds belongs to Allah the Almighty. If he senses any superiority over others, he remedies this by recalling his own shortcomings and Allah's blessings upon him, understanding that obedience is a bounty from Allah. If he fears sins or showing off (riya'), he remembers Allah's attributes of beauty, generosity, and kindness, maintains a good opinion of Him, and hopes for His mercy. Whoever journeys between fear and hope is upon immense good.