What should a young man and woman do to control their desires after they have become attached to each other and wish to marry, especially given the young man's current inability, and their fear of falling into sin?

The work of Da'wah (inviting to Islam) must be in accordance with the Sharia, and a lawful action must be achieved through a lawful means, not by the principle that the end justifies the means. Getting to know and conversing with foreign girls is the beginning of error and a path to tribulation (fitna), and a cause for evil. The Prophet, peace be upon him, said: "I have not left behind me any trial more harmful to men than women." You should completely cut off contact with this girl, except through the lawful means of marriage if circumstances permit. Otherwise, each of you should go your separate ways. To control desires, both you and she should fast, as the Prophet, peace be upon him, said: "O young people, whoever among you can afford to marry, let him do so, for it helps him lower his gaze and guard his modesty. And whoever cannot afford to marry, let him fast, for it will be a shield for him." As long as you are preoccupied with desire, your trial will be prolonged. But when you turn away from it, its evil will vanish. With the help of Allah, you can overcome this ordeal.