/For me, "suddenness" is usually an indication that something else is going on. It may not be a discreet thing that you can name/pick up and throw against the wall so it goes "Splat!" You can only know what is going on for sure over time. Feelings change, ebb and flow and understanding your own rhythm can help so you can tell a bad day, week, or longer period from a, "this is beginning to feel like I may be done, maybe we should talk about that" starting termination period.

I'm so sorry you are having an uncomfortable time of it right now