Likely that she no longer feels useful because every antagonist has drilled into her head that the world "doesn't need the Avatar anymore." Oh, and she's still missing her past lives. And she's wheelchair bound, when she's used to being super independent. And of course, the Airbenders have sworn to take a role in bringing harmony to the world. Further taking over her "role." She's basically been told, and now sees evidence that she's useless. She's likely depressed by all this, hence the tears.
1. That's a fair interpretation, the only issue I'd have is if airbison mate for life. Because at that point, presumably there's some amount of care between the parents.
2. Yeah, I should have kept my mouth shut. My bad. I was a little concerned with people who might think he's serious.