I too am a sensitive person and it took me time to "toughen" up. The key word here is time, it takes time. No one can change their behavior over night. We have to make small changes and stick with those until it's part of us, then we make another small change.
As for the girls at work, don't be so sure that are just able to shrug it off. People can be very good at putting on a brave strong face but when they are alone it can be the total opposite.
Try this: Write on a piece of paper "I am stronger than I know, I have it within me to overcome life's obstacle's"
Tape that to your bathroom mirror and keep a copy in your purse. Read it out loud to yourself several times a day.
After a week or two, add some more positive affirmations like:
"I am worthy of respect"
"I deserve happiness"
"I believe in myself"
"I choose to have Joy in my life"
In order to change ourselves, we have to change our thought process. Again, it's little steps.
