I've been riding for 7 years. I'm barely faster now than when I started. At first I averaged in the 13-14 mph range for rides. Now I'm in the 14-15 mph range. And I have a MUCH lighter, MUCH more expensive bike that fits me MUCH better than back then.
It can sure be frustrating, and literally all my tri friends are so much faster than me that I can't hang with any of them. Most have progressed much better and are consistently riding in the 18-20 mph range now.
Oh well, I got VERY frustrated last season then said to he!! with it. I ride what I ride. Same as running; I run what I run. Even when I was 40 lbs lighter, my running and riding pace wasn't any faster. I am what I am, and now I just enjoy getting on my bike and riding.

It's not always easy to be so blase about it, though. I wasn't for years...