Support ideas and dreams. It is OK for a teen to have unrealistic goals. Also, be there for them. Thou shalt not bring up the past. Bringing up the past doesn't help anyone.
Teens learn from their mistakes, just like anyone else.
And like anyone else, they don't need to be constantly reminded of those mistakes. Thou shalt not be thy teenager's best friend. Too many parents expect to be their teenager's No. 1 buddy. Being caught somewhere in between a child and an adult, trying to find their place in the world and trying to be independent can be difficult, but that is the job of the teen. Teens are trying to keep the security that they have known for the past 13 or more years and, at the same time, gain real-world experience.