quiet divorce

This is a new term so there are some competing candidates — silent divorce and invisible divorce.

It describes the situation in which one partner (mostly the female partner) in a marriage decides that the relationship is doomed and that there is nothing further that they can do to mend it, so they will simply accept that the marriage is over.  But there are reasons why they want to stay in their current situation, most commonly that there are children whose needs should be considered. Sometimes there are practical or legal reasons governing their choice. So they keep their decision to themselves, often assessing a timeframe in which they will continue their quiet divorce before they bring it out into the open. It can exact a mental toll.