How can one restrain oneself from meeting and messaging girls, especially in the absence of parents to offer advice and guidance?

It is obligatory upon you to repent to Allah Almighty by abandoning the sin, regretting it, and resolving not to return to it. If marriage is made easy for you, then marry. Otherwise, strive to fast, for it curbs desire, as the Prophet, peace and blessings be upon him, guided the youth: "O young people, whoever among you can afford to marry, let him do so, for it is more lowering to the gaze and more protecting of the private parts. And whoever is not able, then let him fast, for it will be a shield for him." And you must honor your parents, maintain ties with them, and be good to them.