Will I be sinful if I open the website again after I swore on the Quran that I would not open it?

There is no blame on you for the oath you swore, as long as you are truthful and have no relationship with that girl. As for your oath not to open this website, you are not obligated to fulfill it. Rather, it is permissible for you to break your oath and offer an expiation for an oath, due to the saying of the Prophet, peace and blessings be upon him: "Whoever swears an oath and then sees something better than it, let him do what is better and expiate for his oath." If you break your oath, you must offer an expiation for an oath: feeding ten poor people, or clothing them, or freeing a slave. If you are unable to do so, then fast for three consecutive days.