I can only speak for myself, but there is more to this than just fear of death. It is my eyes being forcibly opened to how evil and stupid so many people are. I've always known there was evil in the world, I've always known that from time to time evil comes into power and does an enormous amount of damage, but I never in a million years could have guessed how widespread and common evil really is. People I love, and who I thought loved me are showing their true colours, as are people I have known for decades. I can't help but look back on my life and think "Was all this fake? The good times, the memories, was it all just bullshit? Were you evil the whole fucking time??" If we were in Germany in the 40s, these are the people who would have sold me out to the Nazis.
Being born on this planet and living through a time like this is like being six years old and arriving at an amazing looking birthday party with balloons and cake and presents and pony rides, only to find out that the cake has been poisoned, the ponies are being beaten with a whip, and your best friend is trying to stab you to death with the cake knife because he wants your balloon. Being at a party with people like that kind of spoils the whole experience.
Being surrounded by evil and stupidity, being betrayed by the ones you love, it makes me feel like all the good times and memories are just an illusion and there is nothing for me here. No point to any of it. So my mental health suffers, just like OP's mental health suffers. I can't see a way out of this. What has been seen cannot be unseen. What has been realized cannot be unrealized. So, it isn't the fear of death that is getting to me. It's living in this fucking nightmare.
Seems like there's something in Uranus. A giant stick perhaps?