Is the delay in conception due to an oath: "If I hit my son, may Allah deprive me of having another"?

What you did by supplicating against yourself to be deprived of childbearing is a mistake and a slip of the tongue. It is not appropriate for a person to supplicate against themselves, their child, their property, or their servants. What you uttered is not an oath, so no expiation is required; rather, it is a supplication against yourself. Strive in supplication and take the necessary means, for Allah is Most Kind, Most Merciful.