It’s not surprising that she doesn’t get it. She had no reason to assume that you didn’t want a relationship, since while you worked together, you, um, kept participating in a relationship.
As awkward as it might be, you owe it to her to be more direct. (“You’ve probably noticed I’ve been more distant lately. To be honest, my life has started to move in a different direction. I’ll always be wishing you well, even if we can’t have the relationship we used to.”) Many people find the idea of a “breakup” nerve-wracking, and – let’s face it – feelings still might be hurt. But I guarantee it’s classier, kinder and more likely to get results than continuing to ignore her.
I’m so sorry. I can imagine your grief and worry. And I don’t get how treatment would be “selfish,” but her idea of not wanting to live a long life is cause for serious concern.
Right now, one thing you can do is legwork. Look for therapists who will work with her insurance and be able to work with her schedule. (Is an employee-assistance program a possibility?)
Keep nudging her toward help, keep listening, keep showing your love, and keep being a positive influence in your grandchildren’s lives.
If she still won’t go, seek counseling yourselves. This will be a lifelong effort – and you deserve support, too.