If things were different then things would be different. (ie never regret, just learn from your mistakes, don't dwell on the past and simply move on)
When someone says something you disagree with then "take what you want, and leave the rest". There will always be people who go against the very core of all that you believe and know. Don't try to change them, just let things be.
No one is perfect, therefore try to always improve yourself and do not force your idea of "improvement" on others.
As we hopefully all know.... advice unsought is advice unwanted.
"Have the power to change the things you cannot endure, and have the power to endure the things you cannot change."
I will add more if I think of any, and this isn't really a life lesson, but I will add it anyways. It is but one small piece of "On Pain" from his most famous work "The Prophet". I do paraphrase a bit.
"Pain is the breaking of the shell that encloses your understanding. It is the bitter potion by which the physician within you heals your own sick self, so therefore trust the physician and drink his remedy in silence and tranquility." - Khalil Gibran
You can find a full collection of his works online, at the library or just about anywhere. He writes on all aspects of life from work to rest, from love to pain. His works have helped me understand many things in life about myself and others. Most importantly they have been invaluable in coping, and understanding with my moms recent suicide.