Though I know the feeling of 'glass is half empty', I feel I don't know the feelings you have {lot of feelings here lolololol}. That said, I have my days in which I doubt everything, in which I think the world is against me in all aspects, in which I get no single thought turned into something positive, in which I am not able to put even a small smile on my face. However, I also have days in which I feel the total opposite like a 'glass is half full' type of mindset. I believe I have more (many more) of these 'full' days than 'empty' days. At least, it's my drive to try everything for that. I even test things like adjusting my sleep rhythms, eating patterns (and content) and even my approach to work to increase the number of positive days, or better said positive moments. Over a decade or more (likely three plus decades) I learned to switch between the 'empty' feeling to the 'full' feeling in a matter of minutes; Not always (well, maybe less than most of the time) am successful when trying, but for sure at times it works.

You wrote about acceptance of those that are dark, those that are in the space of 'glass is half empty'. My worldview is that we all must make at least an effort to accept all kinds of people, regardless of what their beliefs are, regardless of how they view life. Something most of us not even trying to do these days. Many of us live in our own tunnels and when someone or something doesn't fit into that tunnel, we don't want to accept it. But now I'm digressing from the topic of your post. I do hope you'll be able to find ways for living that don't require strong words such as 'constant pain' and 'unbearable' to describe it. Channelling the pain into doing something for others, is to be tried for sure! Am sure you make others happy and hopefully that can bring some happiness into your own life.