Welcome to the forums, Jane. In addition to the excellent suggestions and advice offered above, I would point you to itch.io, which started out as a platform for video games but has a burgeoning tabletop game design community, and the site has actively embraced us and wants to reach out to make tabletop games more a part of the site in the future. You'll find a number of indie game designers who are dealing with very similar content and issues to yours, and you might find it inspiring and maybe even comforting to see what others are doing in a similar design space. You may even feel emboldened at some point to offer your own work there.
so hey, I'm new, and sort of long since traumatized by online communication. so while I know your guidelines for this forum already include being gentle, please be extra gentle with me. I don't really understand humor and joking and generally get defensive when presented with it by strangers.
so what am I doing here if I'm basically socially broken and don't want the normal things people want out of socialization? I'm looking for insight into making a detailed setting for an existing game system to explore themes of loss and acceptance of endings, including one's own inevitable death. the setting would be based off of an original setting I created and believed in as a schizophrenic delusion for several years in my 20's. recently medicated and with a more solid grip on reality I am coming to terms with these delusions being simply that, and attempting to channel the highly structured world I believed in into a creative project as a coping method.
I'm unsure where to begin on the forum. I'm wondering whether seeking help in choosing a base system to make this setting in, and further inquiries that may follow, are best directed to Tabletop Roleplaying Open or Tabletop Roleplaying Game Design. (the latter of which I notice had some specially focused rules that made the general public there sound extra intimidating as a starting point.)
as for who I am as a table top gamer, I've played mainly D20 based games, D&D 3 and 3.5 and later Pathfinder make up the bulk of my table top gameplay, having played in a number of prebuilt settings, as well as improvised settings made by friends. I've also played Mutants and Masterminds, and briefly ran a self fashioned campaign in D20 Modern.
and to close, a random fact about me: I've disappeared into bethesda games over the last 5 years, having more than 9000 hours of combined play between Fallout 4 and Skyrim since 2014