Performing gender

The reason why impostor syndrome exists and even thrives spectacularly among transgender people is because many are worried about being able to adequately, as Judith Butler says, "perform gender". They look at the social expectations and determine that they can't possibly meet them. As a result they are perturbed with being found out or not "passing" (a regrettable term). 

There are masculine women out there who are not worried about being mistaken for men (although some are) and simply live as they desire. The same psychology should instill itself in transgender people which should help solve a great deal of their stress. 

I can tell you with certainty that everything about you will change once this negativity is dealt with and you will be received far better by people.

Is this insecurity understandable? Absolutely; particularly among older people raised under a strict binary. However that shouldn't automatically mean getting stuck there.