Those "me against them" moments you write about, are there for everyone, when it comes to investing ourselves in people who are essentially incompatible with us, because of different value systems, etc.

Just don't let those dictate your inner self-worth because those moments are the ego's response to incompatible personalities and value systems, if that makes any sense?

That's how I frame my own "me against them" moments. When we are in sync with people of like mind, we don't have those "me against them" moments. We are are up against opposing personalities, opposing viewpoints, opposing value systems, then we will have a lot of "me against them" moments.

I think your harsh and ruthless response is a defense mechanism against those incompatible friendships. It makes sense that you would want to protect yourself against other people who put you down for your weight.

Well, those people are shallow for doing that. So, don't internalize their shallow view of you as how you should feel about yourself. Don't compare yourself to anyone. That's a game that can never be won for anyone. It's an easy trap to fall into too. I could compare myself to a lot of people, but if I distance myself, I can see that they have major flaws just like I do, so then, their lives are far from perfect too.