It's important to remember: We are responsible for our own thoughts, feelings, and behaviors. We get into problems when we try to get others to take responsibility for our thoughts, feelings, or behaviors, or when we try to control those of others around us. That is NOT healthy.
All of that being said, sometimes when people react poorly to us, it's hard to brush it off and let it go. Instead, it's easy to ruminate about it, and let the thought fester within us. Here's where the phrase IT'S NOT ABOUT ME comes in handy. When someone has a negative reaction, I need to go back and check myself. Was I responsible for my words or actions? If I was responsible and respectful with that, then that's all that I can do. If someone chooses to respond negatively (i.e. people will perhaps not like the boundary you've established and react angrily towards you), then that's about them and their issues. Perhaps they aren't being responsible for their feelings or actions. It's easy to take their reaction personally, which can increase frustration, stress, anxiety, or tension within you. When we take responsibility for their actions, we aren't having healthy boundaries.
If you've checked yourself and you HAVE been responsible, appropriate, and respectful, then you can repeat "it's not about me" to yourself. This will re-direct your thoughts and will help to let the reaction of the other person go. There's no reason you need to hold onto the negative reactions of others. So remember: It's not about me... and let it go...
What do you do to help let the negative "stuff" from others go?