This is quite a difficult question for me because I made some really terrible mistakes when I was younger but I will choose the one below because of the way it affected my parents, I regret that so much.
When I was in my late teens/ early twenties I became very rebellious and started to take drugs. Now I look back and have children of my own I realise what a terrible time I put my parents through and if I could go back I would tell myself not to be so stupid. It was a bad time in my life because I made myself quite ill, hung about with some horrible people and was generally not a very nice person. Thankfully it was short lived and I came to my senses before anything bad happened to me.
There are other mistakes I made and looking back I wonder what I was thinking at the time but those mistakes made me the person I am today, I am stronger and wiser for them and learned not to do them again. My teens were a troubled time on and off but I'm glad to say I came out the other side unscathed.