It’s really hard when someone dies suddenly. My dad died suddenly, and I was a mess. After the immediate, intense grief period of days, weeks, maybe months, I did think back to lots of times when I could’ve been much more respectful and loving toward him. One of the problems was that I was an only child, so our family was a “triangle” of people and when there would be an upset, often two would take sides against the odd one out. It was wrong and stupid, but we were weak human beings.
Yet even though I was sometimes a little snot, my dad loved me so much. We had our bones to pick, the one that concerned him the most about me (and that we’d argue about) was that I had trouble keeping jobs. It made me really glad that at the time he died I had found my niche and stayed at a job longer than in many years (though that didn’t last forever, but at least he got to live to see it at the time).

The last weekend I saw my dad, I felt an inexplicable urge to hug him and tell him I loved him - I believe this was inspired by the Holy Spirit.
My mother and I had a complicated relationship with a lot of drama, but also lots of love and forgiveness to compensate. The years after my dad died, my mom’s health was in decline. Navigating the nursing home and hospital journey was rough for both of us but also a bonding experience. We had time to reminisce and communicate our relationship and that included apologies and forgiveness for the times we’d given each other a hard time. So when she died, though I was sad, there was much more peace, resolution, and closure.
Over time, with prayer, and maybe special things you do, like visiting the grave, or looking through photos, the regrets should begin to fade and the good memories become more prominent. May God bless you and give you peace in all this.