How can one deal with feelings of admiration for a girl and the fear of worldly temptations and delayed marriage, while maintaining focus on studies and achieving success in them?

If you are able to marry the girl you love, then proceed with marrying her, as the Prophet, peace and blessings be upon him, said: "We have not seen for two who love one another anything like marriage." If you are not able to marry her now, then be patient until you are able, adhere to the limits set by Allah, close the door to temptation, beware of the whispers of Satan, and occupy yourself with what benefits you in your religion and your worldly life.