Is supplicating for oneself—such as "O Allah, have mercy on me; O Allah, guide me; O Allah, grant me provision"—considered selfishness or a sin, while specifically dedicating a general supplication for Muslims and the Islamic nation?

There is no harm in supplicating for oneself with phrases like "O Allah, have mercy on me," "O Allah, guide me," "O Allah, grant me sustenance," and so on. This is not considered selfishness or sin, for the Prophet, peace and blessings be upon him, used to supplicate with similar phrases between the two prostrations. And indeed, there is abundant good in supplicating for the general Muslim community in their absence.