I don't think you'll ever really manage it, ladette - you'll probably end up neurotic either way, it's an occupational hazard as the mum of teenagers.
Mine are 20 & 18, so now both officially adult, 'responsible' for their own mistakes - doesn't actually make any difference to how much I worry, just to how much I have to hide it.
All you can do is make sure your teens have all the information - they will dismiss it with 'yeah, right, you're so paranoid', but it does actually sometimes go in, and just sometimes you begin to get occasional glimpses of a sensible adult in there.
You will worry about everything in the news, their friends, drink, drugs etc - all the possibilities, but I rationalise it by the thought that when disaster strikes, it's usually something you've never thought of, so if you think of everything, it'll be OK....