We keep working

Impostor syndrome is so common among transgender people as to almost be universal. At some points you are going to feel like a fraud and it can often increase in intensity the further along you are on your path. 

The term "passing" is regretable but is related to the concept of being an impostor. You aren't trying to be someone else and yet worry about being detected which can bring up feelings of insecurity that you don't measure up. I don't think it ever fully goes away because we are human and prone to moments of fragility. 

Having been raised differently than your target gender will tend to raise the specter that you aren't really authentic which is only bolstered by people in the world who are all too eager to add fuel to the fire with their transphobia. 

I still fall into these little traps occasionally particularly during this adjustment period where I am settling into a new life as a retired transgender person. It doesn't matter how well you blend in because this about your own psychology. It's not about what others see but instead how, during certain times, you feel about yourself. 

All around It's been getting better for me as time goes on but we all have our moments and so we keep working at it.