There are probably a few things that, if people were to keep in mind, would solve a lot of problems, relieve a lot of stress, and make people a lot happier. I call these the great lessons in life:
Learn what you have to care about and what you don't have to care about - it is not a requirement in life to please everyone you meet; the people who will actually make a meaningful impact on your life are very few, by comparison.
Learn what you can afford and what you can't afford - properly evaluate whether you are able to afford what you are committing to before you commit to it (Luke 14:28-32).
Learn what is important and what isn't important - and be able to let go of what's not important.
What you do to help people doesn't really matter - people who are successful will be successful even without your help; people who are unsuccessful will be unsuccessful even with your help; at best, you can delay the outcome, but unless you keep helping forever, the natural outcome is inevitable.
Don't hesitate to make the right decision - if the outcome is not going to be different, then expedite it; there's plenty of problems in the future that will require your resources.
Accept that not every story has a happy ending - we all die in the end.
Don't take words too seriously - what other people say are their opinions; what you say is your opinion. Only assume a guarantee if it is written and enforceable.
You are in control of your feelings - there are plenty of good or bad things in a person's life at any time; whether you choose to focus on the good things or the bad things is up to yourself. Focusing on the bad things is depressing, but may be what's needed to figure out a solution. Focusing on the good things is comforting, but may allow the problems you face to grow bigger.
In summary, your life is yours to live, and you are the one who will enjoy its rewards or answer to its consequences. So, don't get caught up by what is happening around you, because in most cases, it won't matter and won't be part of your life in the long run. Also, don't try to live other people's lives for them, because that is something that is neither appropriate for you to do, nor something you can do.