Achieving your personal goals in a meaningful way. That’s the internal success. Happiness too.
External success is measured more materially. Your status within society and your achievements. Your material wealth and accumulation, your earning power. The health and growth of your family, how many friends and connections you have. How well you’ve been able to live a life of peace and prosperity.
So you can measure a broad outward success that you can measure pretty much everyone by… Then there is the much more nuanced and harder to measure internal successes that only the individual or those closest to them can really know about.