Where should I start?

At the beginning would be the best place but it would be easier to start with what has bought me to this point.
I have been in a bad place for the past 2 months but it has taken years to get there. Years of just managing to hold on most of the time, with the occasional wobble, which left me unable to function properly for a few weeks or so.
This time has been the worst, no crying, no raging, just nothing. Feeling nothing, doing nothing and being nothing. The mask had slipped and I could no longer keep acting the part of the confident, ambitious go getter. I gave up.
Now I have put the mask back on and jumped back on the roundabout of life and I went back to work on Monday 5th January. It was the hardest thing in the world to get up before the dawn and prepare myself for the outside world but I did it. I even managed to complete a full week at my desk and I haven't achieved that for the best part of the past year.
My GP seems to think that I have had an existential crisis and that talking therapy would help so here I am talking.