I have had the same issues, except with me I either let folks walk all over me, or I go to the other extreme. This has played out many times in my life. Over reacting to things like this has been destructive in my life because it drove people away. In my opinion, these are coping mechanisms that we are forced to learn to survive. I learned to fight, I moved all the time, had no family to back me up and was constantly beaten and RAISED to fight. If I lost a fight as a kid, I had to fight them again until I lost. Sometimes a loss would result in a terrible beating from one of my stepfathers. Learning how to advocate for myself and look after my own interests appropriately has been one of the greatest challenges of my life. I never wanted to be a bully, but I feel I was forced to behave that way in some ways. This is a great challenge for me after the powerlessness I grew up with, we have been CONDITIONED to the kind of responses you describe to people trying to get over on us. Part of that is human nature, but we have stuff built into us through our experiences and family dynamics that guarantee this will be an issue for us. Learning how to 'stand up for myself' appropriately has been a process of taking my power back, and Im not far into it. I've found that real power on a personal level doesn't come from my fists/elbows/feet/knees or even from the barrel of a gun, but from inside me and from spirituality and my ability to connect to it and build/maintain a spiritual life. I am not preaching religion in general or mine specifically (I won't discuss my personal beliefs, generally) Im just saying what's worked for me.
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"what matters most is how well you walk through the fire"
-Charles Bukowski