US /əˈpɑn rɪˈflɛkʃən/
・UK /əˈpɒn riˈflekʃən/
You might discover that, upon reflection, you should change the way you’re doing some things.
You might discover that, upon reflection, you should change the way you're doing some things.
However, if you didn't do anything wrong upon reflection and it started to drain your reserves of self-esteem, that's something that you need to take with a grain of salt.