This is usually attributed to the Diary of Frida Kahlo, which does not contain the quotation. As explained on the Quote Investigator website http://quoteinvestigator.com/2016/01/28/flawed/, a postcard containing the quotation and a portion of a photo of Frida Kahlo was sent anonymously in 2008 to the PostSecret website, which posted a photo of the postcard, but the probable author was Becky Martin (Rebecca Katherine Martin). The actual quotation is: I used to think I was the strangest person in the world but then I thought, there are so many people in the world, there must be someone just like me who feels bizarre and flawed in the same ways I do. I would imagine her, and imagine that she must be out there thinking of me too. Well, I hope that if you are out there you read this and know that yes, it’s true I’m here, and I’m just as strange as you.
Misattributed