How can I be less of a noob at Half-Life?

Let's face it, I suck. I've been playing Half-Life (the original 1998 one) on Steam since around December 2006, and I still get totally slaughtered by upwards of 90% of all people who have been playing the game for more than about two days. I can't aim properly, I can't move properly, and I have no reflexes, hand-eye coordination or sense of proportion. The most common advice I get is to 'just practice', but given that I haven't measurably improved in the last year or so (in fact, in defiance of all known statistical laws, I've arguably gotten WORSE over that time period), that's apparently been just as useless as all the other advice I've gotten so far. I know many other people have managed to get good at this game, and yet I can't see any possible way to achieve that myself; other people don't just move faster than I do, they move faster than I can even THINK. What's wrong with me that I seem so completely incapable of playing well, and what can I do about it?