I have gone through a lot of things both personally and professionally the past years and the number one lesson learned is to not care too much. If every little or big mishap in your life makes you irritated and annoyed then you will never be able to enjoy life.
Honestly after the past years nothing really bothers me. I have realized that it is not worth wasting time on mishaps or people treating you bad, things happen for a reason, learn from them and move on. Like someone wrote the return on investment of not giving a damn is personal happiness, mental freedom and success.
Focus on your goal and not the obstacles you will meet on the way towards it.