How do I deal with deep regret?

For some reason I just can’t shake my grandpas passing, he had his own auto body business and it was pretty successful. When I was a kid I would just play video games instead of learning from him. Then as I got to late teens I was with this girl and spent most my time with her(like a complete dumb ass)My grandpa ended up getting sick and was in the hospital. I spent a couple days with that girl instead of going to him right away because I thought it was something slight and he would be ok. Which was incredibly dumb because he would’ve been there for me in an instant. When I was on the way to him walking up the stairs to visit him, he had passed away. Now I’m here with that regret because I know nothing about auto body work. There’s so many cars left behind that need to be restored and a shop. My dad still works on cars but not really because of health problems. I just have this thought in the back of my head to quit my job, and just try to learn the most I can off my dad because he’s older and I don’t want to live with that regret. I’m 21 but not even worried about trying to get rich anymore because of that regret. I’m sorry for the rant, I’ve never talked to anyone about this lol. I just don’t know what I should do. Shits been kicking my ass for years lol