Advice to a Younger self

I have made many of mistakes in the past. Who hasn't? I try to go everyday with no regrets, no what if i did that? What if i didn't do this? What if i talked to that girl? No one is perfect though and it's almost impossible to live life without any what ifs. If i had a machine to go back in time and give myself advice I would fix so many mistakes.
First time i would go back to is recently. I stopped going over my one best friends house for about a month or two. We were still friends but i just never had time to go over there. His mom and dad treated me like a son. If i had no where to sleep they would let me stay there. If i had nothing to eat they would give me food until i was full. They did alot for me. I was very close with his mom she loved me like a son and she was like a second mom to me. When i stopped going there i never stopped by to say hi to her. I didn't ask how she was doing. I couldn't remember the last time i seen her. If i could go back and give myself advice I would tell myself to go there and see her for one last time. She passed away and I couldn't even remember the last time i seen her and it was very upsetting to me.
Another thing I would go back and give myself advice on is school. From 1st grade all the way to sophomore year of high school i had amazing grades. I never got one C on a report card all those years. My grade point average was 3.5 starting off in high school. I was told my sophmore year if i kept my grades how they are(which in central is pretty easy) I would be able to go to rutgers. My SAT scores were 1200 I was set to go to college. Well junior year i started slipping up and all my grades crashed and that continued into my senior year. I would go back and tell myself to nto slip up and just do the work no matter how of a hassle it is.