Thinking about the things you say isn't faking it - it's just using your brain and being considerate.
If you just blurt out anything with no filter, that doesn't make you authentic, it usually makes you an asshole.
Not all thoughts are productive, many are bad and many are stupid. You should delete those or revise them. Not only when talking to other people, but even to yourself.
I can tell myself that I'm dumb or I'm fat or whatever, but that isn't true and just because I thought it doesn't mean I have to internalize it. No, I filter those thoughts, I tell myself "that's not true". Over time, I think them less.
Your own brain is not reliable. It does not operate on truth, or what is or is not productive. So tune it. Not for the sake of others, but for yourself too.
Being an asshole to others is bad, but being an asshole to yourself is arguably worse. The goal is to, overtime, build better thought procedses and mental models. Not to fake it.