I've made a huge number of mistakes throughout my life, and sometimes I wonder what I'd do if I restarted my life. Most often I'd choose to restart from the point in my life when I was the most happy, that being year 6. I'd try to keep in contact with the people I was friends with in that year more.
Then I'd change my attitude throughout secondary school. Less serious, more fun to be around, and hopefully I'd have made more friends then. I wouldn't have ditched the friendship group I had (although that may turn out to be a bad idea), and I would have been more confident and perhaps apply for university sooner.
If I didn't go to university, I probably would have been more receptive to the people I went to college with. I also would have actually gone to the gym, rather than starting it and cancelling my membership a few weeks afterwards having not gone once.