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

Assalamu alaikum. My father passed away on December 9th from a heart attack. I’m sharing this because I need sincere advice and answers to a lot of questions I have. He had symptoms for about a year that slowly got worse until the heart attack. I was there when it happened and I tried to resuscitate him. At one moment his circulation returned and there seemed to be hope, but then things went badly. For months or even years I’ve been preoccupied with death. I struggle with depression and PTSD, and lately I’ve had a lot of dark thoughts - nights I couldn’t sleep and kept thinking about death. A few days before the collapse I even wrote down those thoughts like in a diary. I feared he might die; sometimes I felt I would go before him. As a child I worried about him because he was the breadwinner, and maybe that fear stayed with me. My relationship with my father was difficult. We argued a lot and couldn’t speak without fighting. Now I regret so many things. I regret not visiting him the week before, not saying things the right way, and not recognizing how much he wanted love and for us to escape his painful childhood. He went through unimaginably hard things and always wanted better for us. We treated him poorly and now I feel guilty. I have some specific questions and concerns: - When I make duas and recite surahs, sometimes I get a strange feeling - like someone is behind me watching. Once after reciting Al-Fatiha and saying Kullu Allah three times I felt that presence. Other times in prayer I’ve felt the devil near me. Is that normal? Could it be my father, the devil, or just my mind because of grief and PTSD? - Is it normal to have had those long-standing feelings of dread and preoccupation with death? - From an Islamic perspective, what should I be doing now? I know I should pray more, make dua, and read Quran, but depression makes it hard to find motivation. I do manage sometimes. What else is recommended - for my own heart, for my father’s soul, and for coping with guilt and grief? - About the medical side: his doctor didn’t take his complaints seriously, attributed things to a slipped disc, and didn’t refer him to a cardiologist despite a history of heart problems. My family wonders whether we could seek legal action in Germany with a good lawyer. Is that something we should pursue? I don’t know what to do. Please give me practical advice: what should I do now, how to make amends for things I regret, what Islamic acts are best for a deceased parent, and how to handle the medical/legal questions. Any comforting words, steps to take, or dua suggestions would be appreciated. Jazakum Allah khayr.