So for me, nothing mattering is related to depression. How that relates to what people call Nihilism, I'm not sure.

I did not relate "nihilism" to depression. It was not my clear intention to do so. If it were so, I would have visibly talked in my post about "depression" linked to nihilism.

Though I - after you talked about it - confirmed it to be true that in such a depressed state of mind the thought that "nothing matters" is very dominant, depression in relation to nihilism was not part of my given context.

I followed what you believed I was talking about and added some of my thoughts on the subject as I went along.