Inna lillahi wa inna ilayhi raji'un - My dad passed away

Assalamu alaikum. My father passed away on December 9th from a heart attack. I’m looking for advice and answers. He’d had symptoms for about a year that slowly got worse until the attack. I was there when it happened and tried to resuscitate him. For a while his circulation returned and there seemed to be hope, but then things went badly. Inna lillahi wa inna ilayhi raji'un. I’ve been struggling a lot. For months or years I’ve been preoccupied with death and had frightening thoughts and fears. The night before his death I couldn’t sleep and had very dark thoughts. A few days earlier I wrote down diary-like thoughts about death and even cruel ideas like “why don’t those who aren’t needed just die,” which makes me ashamed. I also have depression and PTSD. My relationship with my father was very difficult. We argued a lot and couldn’t speak without fighting. Now I regret so much. He had a very sad childhood and spent his life trying to protect us from that. He wanted love and we failed to give him what he needed. I didn’t visit him the week before he died and I regret it deeply. He worked and supported the family and that always weighed on me as a child. I thought I had more time to say things properly; nobody told me the full truth about how serious things were. A few things I need help understanding and practical guidance for: - Dreams and visions: When I dreamed of him most times he was angry, blaming visitors or other things. But one dream was different - we hugged, forgave each other, and he said he loved and missed us and would come back. From an Islamic view, what might these dreams mean? Is the forgiving dream a good sign? What about the angry ones? - Spiritual feelings: Sometimes when I say duas or recite surahs (I’m Muslim), after reciting Al-Fatiha once and “Kullu Huwa Allah” three times, I felt like someone was behind me watching. In prayer once I felt like the shaytan was with me. Could these feelings be from grief, from the shaytan, or my father’s presence? What should I make of them? - Is it normal to have had this ongoing feeling about death and to be so fixated? I thought I might die before him, feared losing him for a long time, and now it all happened. - What should I do from an Islamic perspective? I know to pray, read Quran, make lots of dua and Istighfar, but depression makes it hard to keep consistent. I manage sometimes. Are there other religious practices that might help - specific duas, surahs, or actions for the deceased and for my own heart? - Medical/legal question: The doctor didn’t examine him properly, dismissed symptoms as a slipped disc, and didn’t refer him to a cardiologist despite a history of heart problems. Could we consider legal action in Germany against the physician for negligence? Any practical steps to take there? What I’m doing now: I try to pray when I can, and I sometimes recite Quran and duas. But motivation is low. I feel guilty and overwhelmed. Please, from a Muslim perspective, what should I do for my father and for myself? Which duas or acts of charity or sadaqah could help him? How do I cope with the regret and the dreams and the uneasy spiritual feelings? And any advice about pursuing accountability for the doctor in Germany would be appreciated. JazakAllahu khairan for any advice. Forgive my imperfect wording - I’m exhausted and grieving.