You are allowed to be in the world. You are allowed to make mistakes. Trying to be a perfect person, to anticipate any shortcomings or absentmindedness beforehand, will do nothing but continue distressing you and alienating you until it's too late.
Instead, do your best to be self-aware and mindful. Do the damn thing if you still want to. And if you realize your mistake after the fact, correct it. Apologize, take it down, throw it away, address the feelings that brought you there originally, and do your best to commit to being better about it in the future. That's it. You are allowed to figure things out as you go and be witnessed in your errors. You're not evil or valueless because you make mistakes, even obvious ones, or because somebody sees it happen.
We're all only human. We're all figuring it out as we go. Be kind to yourself.