The letter to my 100 year old self would comprise of these three tips for longevity as a ready reckoner for the young generation.

The three golden tips for longevity would be as follows:

1. Always putting Long term Fulfillment (Oxytocin) over short term Excitement (Dopamine)
2. Appreciating that Life is an Infinite game
3. Practicing regular Gratitude Visits

These three things when practiced on a regular basis would not only make our lives fulfilling but enriched as well.

Read these insightful blog posts highlighting in greater details the above mentioned three points:

https://viewpointsunplugged.com/good-health-demands-fulfillment-over-excitement/

https://viewpointsunplugged.com/practice-the-attitude-of-gratitude/

https://viewpointsunplugged.com/keep-playing-the-infinite-game-of-life/

The above points cover the mental aspects. Equally important are physical aspects such as well balanced diet and regular exercise.

https://viewpointsunplugged.com/why-choose-your-diet-wisely/

I would highly encourage you all to read this book from Simon Sinek to slow down and focus on winning the war rather than needless battles.

https://www.amazon.in/Infinite-Game-Simon-Sinek/dp/0241295599