I've often heard "projection" used for this colloquially but from what I can tell the somewhat formal definition of "projection" seems to require doing so as a defense mechanism such that you deny that you feel the way you do while assuming that others feel the same way as you sans the denying part.
Is there a word for this notion without the self defense part, that is more or less free of any kind of value judgement on the part of the person holding the belief or the person(s) assumed to be holding the belief, and free of any kind of intentionality, e.g. I want you to believe what I believe?
Sorry, I'm ____ again. I shouldn't have assumed that.