When I was younger, I cared too much about what people thought of me. This affected me whether I was doing something good or something I shouldn't have been doing. I spent too much time overthinking and overanalyzing what people would think of me. Then one day I realized that people don't care: it's not the big deal I made it out to be in my head, or it's what everyone does, or there are things far worse than what I did. This gave me the freedom to stop caring so much about how others saw me and what I did.
Another thing which allowed me greater freedom in my thoughts and actions was that it's OK to be a heel. Every now and then a bad guy is truly needed, and someone has to play that role. It could be that someone has to say "no"; it could be that someone has to reveal the cold hard truth and it hurts; it could even be someone who has to cut the party short. This sort of heel or "bad guy" isn't the criminal type we see in movies, but it's the kind of person who isn't there to be a "best friend" or pushover but rather someone who keeps it real for you. Maybe it's age, but I find that older people are more open to embracing their roles as heels.
When we're with people of a certain age (usually 18, but it can vary), we cannot tell them what they should do. The best we can do is offer them options or alternatives. ("I don't agree with what you're going to do, but if you insist on doing it then this is how you go about doing it...") We can explain decisions and the likely consequences of those decisions. After that, whatever happens is what happens. Should things go wrong, we may give a gentle reminder before offering support and comfort to deal with a setback; then we move on and look forward to better times.
The main thing in all of the above is to not go crazy over things. For things within our control, we don't need to worry because the power to decide is in our hands. For things beyond our control, we don't need to worry because all the worrying we do has no impact on what happens, so there's no reason to waste time and energy on it. Instead, we just take things as they come, one at a time; we also treat others as we wish to be treated. Just as we want others to accept us on our merits, we need to accept others on their merits.