Everybody picks up bits of insight and inspiration during their journey. Here's a short list of what I think makes for a fulfilling life.
This is in no particular order, and I occasionally add to it as I see fit.
Most of this falls under "what would I tell my younger self" category.
What you don't do doesn't matter (hat tip to CG)
People can transform.
You can change how you live, how you eat, and your physical and mental health through deciding to. It may be hard, it may take a long time, but if you want to change any part of your life, habits, or personality, you can (at any age). The key is deciding to do it, not just hoping for it.
Time spent wasted is the biggest detriment to a fulfilling life.
By "wasted time" I mean living without larger goals, hopes, dreams, and purpose.
By "wasted time" I mean not trying to improve yourself in incremental ways continuously.
By "wasted time" I mean hanging out with people (or working for people) that don't add anything beneficial to your goals and your life. People that bring you down or try to put you in a box or control your thoughts or your time or "tell you how it is" are vampires that should always be kept away from. Run. Fast. They are trying to define their lives based on controlling you.
Time spent thinking, dreaming, wandering, and planning is never wasted time.
Time spent loving family and laughing with true friends is never wasted time.
And time spent creating is the best time of all.
The biggest thing to avoid in life -- treat it like a deadly virus, so you're nowhere near it -- is debt.
The biggest thing to embrace in life -- run to it, seek it out, or build ways to surround yourself with it -- is optimism.