> what percentage of my life is already gone by

It's even worse when you consider your perception[1] of how much of your life has gone by. At age 20, you perceive that 50% of your life has gone by. That is, you feel like time passes equally slowly (or equally fast) between 0-20 as between 20-75. At age 40, you perceive that 75% of your life has been lived.

[1] I don't recall the source, but it was a psychology or neurology article. I don't claim that the study is flawless, but it does feel to me like there is merit to the argument that a decade in your youth seems like a long time but a decade as an adult passes quickly.