I was declined an ESTA due to answering yes in have you ever been convicted. I thought it would ask when and why to explain but no its a blanket question. I was a junior aged 16 and only received a minor police caution for possession of a controlled drug, I am now 43 and never been in trouble as an adult and I am fully SC cleared for my job.... what is the best approach?... I assume going to Canada is out the question, what a mess.
ESTA materials say that if your application is unsuccessful you should apply for a US visa. That is where you will have the opportunity to say "when and why and explain."
I suspect your chances are fairly good, but if you are not successful you will lose the $160 application fee.
The best approach is now to apply for a regular visa. AFAIK there you can explain a bit more about your circumstances in the visa interview.