Who we are today, is a result of the life experiences we have gained up to this stage in life.
Too many dwell on what could have been if their situation had been different than it was but in actuality, hindsight is 20/20 and no one really knows at the time what the future will bring. Most 18-year olds typically want to party and chase the opposite sex and think there is lots of time ahead of them.
You can't change what is in the past. Better to utilize your gained knowledge and experience to recognize what not to do so as to focus on changes to make over the rest of your life.