How can I apologize to my neighbor whom I wronged 10 years ago? Is repentance sufficient, or must I apologize directly, or can someone mediate?

If the injustice pertains to honor (such as backbiting and the like), and the oppressor fears a greater harm by informing the wronged person, then for repentance, it suffices to: cease the sin, feel remorse, frequently supplicate for the wronged person, seek forgiveness for them, and praise them among people. It is also possible for a trustworthy person to mediate and convey to the wronged person the oppressor's remorse, apology, and shyness to meet them.