Introspection is key. Are you actually nice or do you just think you are?

Would an average person describe you as nice? I'm not saying you should care what others think, I'm saying you can use what others say as a starting point, especially if it's someone whose opinion you respect.

I don't care if racists or sexists call me intolerant, but you can be sure that if my best friend who is a very sweet person said I'm being hateful then I would take that to heart and reevaluate my biases and preconceptions.

The lesson you needed to learn there was that you are surrounded by people who take advantage and you need better friends. It's like making a peace deal with Hitler and being surprised he broke it. Of course he broke it, he's a terrible person. You need different allies.

The golden rule is actually 2 rules:

1. Treat people the way you would like to be treated.

2. If people treat you the way - you wouldn't want to be treated - you're allowed to treat them the same way back. Or to just walk away...

Seems like you need to learn how to set boundaries.