To be sure some of it is probably in each of our indivdual bodies - some people are exceptionally physically gifted and will seem to make effortless strides, some people can work their butts off and make only small gains. In the end you are doing something that is good for you and that is what counts.

Some is certainly in the equipment. It doesn't have to be the newest, flashiest, lightest stuff around, but it does have to fit properly. Little things can make a big difference and if you don't have just the right fit you probably will go slower. Some things are meant to be taken slower too. Don't expect to a touring frame to be as fast as a racing frame- the tourer is built for long haul comfort and stability. I used to be stuck at around 15mph, really pushing to make 18, plus I was a terrible climber until I got a new smaller bike. First off I saw an immediate change in the speed I was able to achieve and the hills I was able to climb and second off I started making pretty fast progesss that it seemed like I was not making before. Some of the progress may have come from increased riding, but I started riding more because I was making more progress. It really feeds on itself.

My advice
1. Don't give up - no matter what you are doing something that is healthy and you are still doing something that many many people don't do. Sure you can say all the people in my club are faster than I am, but then think about all of the other people who think that 30 miles on a bike is about as achievable as the Ironman for them.
2. Try to not compare yourself to other, people. If you need a comparison try seeing where you were last year, last month, last week.
3. Make sure you really have a good fit on your bike
4. Don't get so stuck on the numbers that you stop having fun