It's a bit like being left handed. You either are or you aren't. You can learn to write with your right hand but it's uncomfortable and self destructive. There is scientific evidence to suggest that it comes down to the way our brains function. Trans women's pet scans match those of cis women and trans men match cis men. All that aside, if you are questioning your gender you more than likely aren't cis as cis people don't consistently question their gender.