DS
It is called autogynephilia - look it up. You are basically short-circuiting your sexual arousal pattern in your brain by finding the "perfect woman" in the mirror and not in someone else who comes with emotional baggage and the stress of a relationship. You know the girl in the mirror will always look the way that excites you most and will always play along with your fantasies. There is no fear of rejection or judgement.
You get to play both sides - the desirer and the desired, so arouse yourself by looking at yourself in a female form. It is a closed, perfect system of arousal which you can maintain because you only dress in the closet and do not present yourself as a female to other people giving them the opportunity to interact with you and pass judgement on you.
So if you wanted to break out of this perfect circle, go out crossdressing in the real world so you have to deal with the stresses of being a CD and having other people accept as such. If you were to find a GG who did accept you then you would likely find the same passion would continue with her instead.
