I have two main ones that are sort of connected.
First, I regret not finishing college. Right out of High School I took everyone's advice and decided to go to the local community college, which happens to be a very decent one. I registered, and not only was I accepted but because I placed exempt out of every subject in placement tests I was offered a full-ride scholarship without even asking for it. I was extremely fortunate and proud and excited for the future. Unfortunately, I also was in the midst of my first real relationship I started with a girl 85 miles away right after I graduated, before I started college. Long story short, she was from a very poor family in a small, shitty town and going to see and spending time with her and her family was very time-consuming...and like a typical small town, low-income family it came with a lot of issues and drama. She was 2 years behind me in school and the first year I juggled being with her and her family and College relatively well. Grades were good, not perfect and I was able to keep the girlfriend from dropping out like her family wanted her to. After the first year, I was still living with my parents and she with hers and my parents suddenly decided to move to Colorado. I took my girlfriend with us and being with her and living with my parents while helping to make sure she graduated high school (against her will) completely took all of my attention and College just slipped through the cracks for me and that was that. No more College.
That brings me to my other main regret, becoming trapped in the past. Now I have been 10 years out of my first year of College and have made little progress in my life relative to what is considered successful. Haven't been with or spoken to the girl for 6 years that I threw away my education for. I am married to someone else now but my life since then has been a roller coaster of moderate success/financial independence and utter failure and loss of home and treasured belongings. Every single day I live lamenting better times in the past, friends I have had no contact with for years and mentally reliving all of the moments that have shaped my life. I play the same video games over and over year after year because I enjoy them immensely but they also are a way of reliving the past. I do the same thing with TV, which since I rely on my computer and not cable TV give me content I pick everything I watch. Every time I have to pick something to watch or even have on in the background, every thing I consider I also end up remembering some time in the past when I watched it. it's like going through a table of contents of my life and just brings me down quite a bit.
Minus a bunch of other lesser regrets and life events, those would be my main 2 and they are the crux of my life since High School.