I have lists of things other people have said to me that are positive. I email them to myself and look at them. Generally, if someone has said something negative to me, someone else has said something else positive to me about generally the same thing.

Also, I give it time. If someone says something that offends me once, okay, maybe they just had a bad moment. I wait to see it's "true" by waiting until the next time I interact with the person. If they have forgotten all about it, then I probably misinterpreted what they had said to me.