Ditto to what manuva said...for years thought this was a problem of mine, then I sacked up and hit the gym. After just 6 weeks of back work it never was an issue again. Confidence shot up dramatically.
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Okay insecurities...
I also live with my parents (a sacrifice of temporary comfort while I build my career and pay off debt), but that doesn't bother me much because I'm doing it for completely valid and good reasons, as opposed to a lot of bum twenty-something children these days.
I'm insecure about my past. I was a complete geek in high school. Didn't play any sports. Sent women running for the exits with my puny, nerdily arrogant, poorly dressed, socially unintelligent, extremely booksmart, and highly masturbatory ways. Never participated in any of the school dances or other traditional functions outside of band. Throughout college my friends were party animals. I got a little woman love from some nasty hoes when completely drunk, but never dated or anything. My friends weren't people of any substance. No work ethic at all. They were the whiners, stoners, and retired high school jocks who will one day realize than fifteen years of living for binge drinking, weed, and the strip joint has fvcked them for life.
Fast forward to today and I'm such a radically different person. I've hit the gym hard, eat well, and am in great shape. No more drugs, smoking, or binge drinking. I've fixed all the major issues with my inner game. I've shed myself of all the losers I've formerly associated with, which unfortunately has left me with a very small social circle which I'm working to rebuild properly. I've worked my ass off and my career is on the rise. I'm dating an attractive woman who's classy, intelligent, fun, low maintenance, and traditionally feminine. She cooks for me, cleans up after me, the sex is awesome, and there is absolutely no oneitis or "relationship agenda" on either of our parts.
No doubt, being on the loser side of the coin for so many years made me a stronger person, and a side benefit of being a geek back in the day is that I'm extremely well-read and educated. But to bring this post full-circle, my insecurity is the possibility that the people of my future might become aware of my past and view me poorly because of it.