Advice for gay nerdy virgins without a ton of confidence?

I'm 23, and I've never had any relationships, straight or gay; I've come to find over the years that even though I'm interested in someone, I hold myself back and just assume that I'll never be handsome/tall/successful/hot/good enough for him. Lack of confidence is a problem for me, and it's not something I can acquire instantaneously; it's also probably a self-fulfilling prophecy. Whenever I see or meet someone whom I'm attracted to, my first thought is that I'll never have any real chance with him, and I simply don't know how to get past this self-imposed obstacle. I'm also incredibly afraid to go to the gym: I don't know how to use any of the machines, and I'm nerdy and I don't feel confident enough to traverse into an area of physical competence. I can't help but compare myself to the people around me and think that I'll never be successful or handsome or "good" as them. I'm only 5'6" and I'm quite skinny, and I feel that nobody takes me seriously in any form of encounter, whether it'd be of a professional, academic, or sexual nature. I'm quite sociable, but, in my perspective, most people simply dismiss me as unimportant or irrelevant upon first sight, without any regard whatsoever to my personality or background. I've read books about the psychology of physical self-presentation, but the tips don't seem to work for me.