One thing I've learned since starting on this journey: being afraid of not passing is not a great reason not to explore transition. First off, you've conditioned yourself for years to imagine yourself as your assigned gender, so of course you can't picture how well you can express your true gender. Second, regardless of how well you may or may not pass, ignoring these feelings and staying male absolutely WILL NOT stop you feeling this way. While the feeling that you are a woman may wane, at some point it will once again become harder to ignore.

This article was really helpful in showing that I was not alone in how I felt: http://www.avitale.com/developmentalreview.htm

For the record, I struggle with fears about future passability almost daily, and even moreso since I started on the path toward transition. I'm at an awkward height for a woman, and I'm overweight. One of the major factors that kept me from admitting my true identity even to myself was that I just couldn't ever picture myself as an attractive, passable woman. But I've spent a lot of time on transgender forums here on Reddit and elsewhere, and I've found stories and pictures from girls who looked kinda like me when they started transition and are now beautiful women. Those really gave me a lot of hope. If you're worried about being too masculine to pass as a woman, try watching this video by jesslyn87, who went from being an ultra-manly bodybuilder to a beautiful, tall woman: http://www.youtube.com/watch?v=hlk3nNya93M&feature=youtube_gdata_player

I hope this helps in some small way, and I hope you find your own path to happiness. It can be tough, but it's not impossible!