re: Has anyone here dealt with the onset of mental illness with their spouse?

What kind of mental health issue are we talking about?

There’s a decent amount of disorders that have onset in 20s and 30s, so if you’ve been together since you were younger it’s entirely possible it didn’t emerge until later. For a fair amount of personality disorders, they aren’t typically diagnosed until adulthood as well just due to other potential things that can contribute to emotion dysregulation like hormones in puberty or environmental factors (parents with poor emotion regulation).