I very much agree with this. The current financial and property climate makes things very difficult. Obviously it works in some cases but generally I don't think there is much to be gained from an extended adolescence and essentially putting off getting to grips with independent living. Some young adults need an extra year or two to be emotionally ready but other than that, it's hard for parents watching their adult DC make mistakes without feeling the urge to step in, and it's hard for DC not having the freedom to be young and live their life how they want to live it.
Op, in answer to your question, the answer is be a bit "selfish" and get busy with your own life. Please yourself a bit. Focus on things that bring you calmness and fulfilment and good health. Start those hobbies you have been putting off. And let your teens suffer the natural consequences of their behaviour. Obviously you need to keep lines of communication open and step in if you sense serious danger, but otherwise, leave some bowls of healthy home cooked food in the fridge three times a week and let them get on with it, while supporting very much from the sidelines.
Instead of saying "I am pissed off with you for sleeping through that job interview and you had better not do it again or I will take half of your allowance" maybe try "I will be very interested to see how you choose to approach this next interview" putting all the responsibility back on them.
Don't be tempted to bail them out if they run out of money, or miss that important lecture because they had too much to drink the night before, or don't have clean clothes because they failed to do the washing. Don't offer advice on relationships unless asked but be there to console when it goes wrong. Don't smooth their paths too much or sweep up after them either. This is the only time they can make mistakes and you be there as background support, so let the mistakes happen.
I hear you over the frustration of not making the most of opportunities coming their way though! Gah! So frustrating! They don't recognise them to be as important and significant as we do because they don't recognise their own privilege sometimes, or fully understand how hard it is out there, or they are too terrified to find out, and they just don't know how everything works yet. That takes time to develop. I look back at some of the opportunities I failed to pick up on, not recognising them as such at the time, and I cringe with embarrassment at my former self 😃. I did speak to my teens about my cock ups though.
The best thing you can do op is to prioritise your own well-being and be a good role model, demonstrating purpose, self-discipline, resilience, relative cheerfulness, with a proportionate level of anxiety or concern, kind of give them a blueprint about how to face challenges and live as a balanced, evolved adult, because even though they do not appear to be listening, they are definitely watching, and hopefully some of that will rub off! 💐