My dad died this month. I’m the eldest of 3 kids. My youngest brother died suddenly in his sleep in September 2019 due to arrhythmia at the age of 25. My dad and him were extremely close, like extremely extremely close, to the point where if you had to convince my dad of something, you’d have most chances through my brother than through my mom. After my brother’s death, my dad became a total recluse. Quit his job, moved to a secluded place, devoted himself to spirituality, build one of the most beautiful mausoleums I’ve ever seen for my brother - which he’d visit every single day. My dad was diagnosed with acute myeloid leukemia last month - he had 0 will to live, staunchly declined any treatment except for hospice. When he died I felt a bit sad naturally, but glad is how I felt the most, relieved: I know he didn’t enjoy living anymore. Physically he died this month, but in most ways I feel like he died back when my brother died. Every day was a struggle for him. I feel like he’s finally at peace now.
What you’re describing sound more like relief than happyness.
I felt somewhat similar when my dad died. He was drinking and declining mentally. Of course it saddened me, but with us having our first born at the same time, it was also a huge relief not having to deal with his issues in that situation.
My mom started developing dementia about a decade before she died. 5 years after we first noticed the changes, she could barely form a sentence. That is when I mourned her because that is when I lost her. When she finally died I felt mostly relief. I will always miss her but I was happy when her suffering finally ended.