@Mr_NutterButter the thing that immediately jumps to my mind is anxiety or dread, in more simple terms. but those words both carry more actively negative connotations, rather than ones that might be either melancholy or somber.

i'm also reminded of "memento mori" which means "remember that you will die" but it does carry this same kind of feeling you describe. you know you're going to lose your life eventually, you may not know when or how, and you may have even accepted that you will die, but it's something you know will happen and all you can do is wait.

i'm not a big latin guy by any means, but maybe something like "memento amitto" could work.