I find that most people who tell others to "Be themselves", are usually the most interesting and authentic people; I rarely hear it out of the mouth of the ones who follow crowds and conventions. Those people -at least over here- tend to say "act normal", by which they mean don't act strange, 'cos strange is scary. I find it comforting when I meet people who tell me to 'be myself', although I almost immediately let them know that the advice is unnecessary, because I already walk my own road and am very happy about it.
But, as for the problems that may be caused by being yourself, tweety-fan, I have asked myself the same question you have. If being myself means acting autistic or being autistic, then wouldn't I just be incredibly awkward, clumsy and unhappy in my daily life? To which the answer for me turned out to be, 'no, not at all'. Sure, I am and always will be autistic, but I've been able to look at myself and my autism rationally, ask myself which aspects of me as an autist could be improved on to operate much more smoothly in this world, while retaining those traits that are obviously autistic but in no way negatively affecting my functioning. I mean, if I prefer to isolate myself sometimes, if I prefer not to talk to others, or if I feel I need to stim, who is going to stop me?
It's an important thing for both you and the people around you to remember that you're autistic, but you're not autISM. It's not the be-all-and-end-all of your person... but being yourself will -perhaps for both better and worse- always include autism.
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clarity of thought before rashness of action