Oh Darling i know that feeling. I did NOT live a pristine life when i was younger - i did a lot of what my son does - i just never got caught and was very lucky that it never affected my health, freedom, life. So now that my son does these things i look back and wonder, who am I to judge what he is doing? I went too light on consquences because of my guilt and ended up enabling him.
So here's the thing, its not judging him and its not the example i set because it has NOTHING to do with me and everything to do with him - the fact is I am his parent and as his parent i have to keep him safe and teach him right from wrong. We know its wrong not only because that's what all the professionals tell us but because we experienced it ourselves firsthand. It is hard to see your child walk the same path and they will throw it in your face as soon as they figure out that trick but it doesnt matter. You are clean now and you know what you have to do as a parent. Sometimes being an open book when they are young is not the best choice - sometimes you might want to hold back some details until they are older and can understand it because at the young age they sometimes justify their behavior by saying "well my mom did it." if that's too late for you (as it is for me) then just let it go - its in the past and nothing you can do about it now. but you are mom and as mom you have a responsibility to teach them the dangers of this behavior and you have a responsibility to punish when you feel necessary. Guilt will get you nowhere and does them no good - you are better able to teach your children about drugs and alcohol because of what you went through - just take what you have learned and teach them well. If it makes you feel better my son says that the best counselors are the ones who used to be users - the ones who never used he says cannot understand him. So in a weird twisted sort of way you are actually more equipped to help them through these tough teen years. Be thankful you are not naive as that would be much much worse.