I have been hearing impaired since birth due to nerve damage. Since the problem is with the nerves themselves, there is no treatment so there's no hearing aid or surgery that can help.
I read lips to some extent, and that combined with my remaining hearing, and positioning myself where my good ear is to people and they are in my line of sight gets me past almost all situations so most people don't know.
The problem comes with the remaining time such as when someone is behind me or on my bad side, rooms with plenty of conversations or poor acoustics and I have to let people know, as I have not responded to people in the past and they thought that I have been rude or ignoring them.
Since this is an effectively "invisible disability" that limits me only when I can't compensate for it, how do I broach the matter diplomatically? I don't want sympathy or pity, but I don't want to be seen as rude or cold either.
What complicates matters and makes a simple "I'm sorry I'm hard of hearing" difficult to say is that in most cases people don't get the fact that I can "hear" them when I'm looking at them but suddenly not when we are not facing each other.