Look, dealing with a mental illness can be challenging whether it's yours or someone you love. It's not about never having a bad day but it's about getting through the bad days and enjoying the good ones. When the inevitable bad day is over, start enjoying life immediately.

You might have to take charge of that change. I was with someone with DID once and I found that humor would often work. Something like:

"Ahem. This is not about blame. This is about love and getting through it together. Things got sh*tty, we got through it. That's to be celebrated not suffered over. So no wasting today feeling guilty - if you want to waste the day you can give me a foot massage till you feel better. "