I would say that "reacting" is anything that you do in response to something external to you. It would include feelings that you hold in and don't express, as well as outward displays of anger. People with avoidant personality disorder are very sensitive to rejection, so much so that they avoid social situations where they could experience rejection. That could easily be a reaction to having been hurt by criticism and being sensitive to it.

It's very hard to change, but I have often heard or been told that people are to act and not to be acted upon. To react is to be acted upon and to let that decide your actions (or lack thereof). If you see that in yourself, maybe you will want to work on trying not to let what someone else says or does determine how you feel, especially how you feel about yourself.

Rap