Sorry you are going through this, and left alone with the news. From what you have written, some of the expressions, she has used, sound like, there may have been built up resentment through the years.

It's not entirely, uncommon, for couples to stick it out, in marriage, for the sake of children. It might not, entirely be a late life crisis. It will be rough on all of you, involved.

She's a grown woman, making her own life choices, at this point. I hope, you have your own support system through this. Sounds like, she trusts you enough, to express this to you. Sometimes, grown moms, just need a supportive friend, in their grown children.