Teach. Find people to mentor, find user groups to present to. There is no better way to learn more about subjects you're superficially familiar with than to teach them. Pick subjects where you think you're an expert, and prove to yourself that you weren't, or pick subjects that you would like to know more about and try to understand them well enough to explain to someone in 45 minutes.

You will be terrible at this at first, but teaching is also a skill that takes lots of practice to acquire.

I would also agree with what others said: stop comparing yourself to others, no good will come of it. There is always someone better, there is always someone worse. It doesn't matter. Set a standard for yourself and meet your own standard.

P.S. If you're looking for a good resource to learn presenting from, I would recommend macsparky's presentations field guide.