How frustrating for you! Don't you just love people that seem to be on another planet when you're trying to reach them?

Have you tried telling her "Mom I need to tell you something and I need you to focus, ok?" If she changes the subject, repeat "Mother! Focus!" but do it with a smile on your face.

Try giving her "just the facts, Ma'am" and keep away from emotions as much as possible. The emotion part may be what she has trouble with. Maybe! I don't know, but it's worth a shot.

Good luck!