A. Ready and willing to acknowledge personal failings in an effort to mitigate them.
B. Stubborn and in denial, but a painful moving on and the subsequent self reflection will get them on the right path.
C. Extremely stubborn and they may go their whole life blaming others for their failures.
It sounds like you fell somewhere between group A and B when you were younger. Life pushed you into group B when it might have been possible that a mentor could have helped you be in group A. It is also entirely possible that you were firmly in group B at that age and the 'wasted' 12-18 months was actually just a necessary learning experience that school could not provide.
For the moment let's assume that there is an equal chance they are an A, a B, or a C. Talking to A could help them if everything goes right. Talking to B would be at best pointless. Talking to C would probably backfire internally. I don't like those odds.
If you are confident that talking to them will help them and that it is worth the risk, don't tell them anything. It sounds like the improvement they need must come from within. You can ask them general questions that get them thinking in the right direction.