First - what are boundaries? Boundaries are like the foul lines on a baseball diamond. Boundaries are what give us separateness from other people. You are here & they are over there. Imagine a baseball hit outside of third base. Because the ball went over the line, it is considered a foul ball. We must have personal boundaries as well. Until you internalize this separateness from others, you won't feel safe in the world.
Another person's thoughts, attitudes, beliefs & feelings about you can't touch who you really are. No one can change your selfhood & identity by being hurtful to you. Not being honest about your boundaries makes you feel betrayed & angry toward others, as well as yourself.