The fact that learning to ride well takes so darn long drives many an intelligent riding student crazy. They've been taking lessons for years and somehow, while hopefully having made some progress, quite often still feel inadequate in the saddle. Yet in other areas of their lives, they're overachievers. What is it? Lack of talent? The wrong horse? The wrong instructor? No, ladies and gentlemen. Math!
Firstly, dressage is an incredibly complex sport. And it involves a horse (obviously), but the implication here, beyond the fact that it comes with a mind and body of its own, is that it is a living being whose needs must be placed first. And that limits, amongst other things, practice time. While you might feel like you'd like to practice lateral work for another hour or two, your partner's soundness (and attitude) would suffer in the long run, and hence extended practice sessions are out of question. While there are a lot of things you can do out of the saddle to support your riding, such as studying the classical literature or working on your fitness and core strength, riding is ultimately only learned by riding.
You may have heard of the "10,000 hour rule", the idea that mastery of just about any subject takes 10,000 hours of deliberate practice. Ideally with a good coach and presupposing a baseline of talent (i.e. just 10,000 hours of any practice don't lead to greatness). This goes back to a 1993 paper by Anders Ericsson titled The Role of Deliberate Practice in the Acquisition of Expert Performance. The idea was later popularized in Malcom Gladwell's 2008 Bestseller Outliers. The Story of Success.
You see where I'm going with this. Let's say dressage "mastery" is achieved when you can train a horse to Grand Prix according to the classical principles and you could be competitive (whether or not you choose to show). Let's furthermore assume you're the average amateur with one horse. Even if you rode five days a week under the supervision of a great coach, that would only be 260 hours a year. And for most adults, 5 days a week is pretty unrealistic, and it also presumes that neither horse nor rider have to take any time off. But let's just say you can be that committed and get this lucky at 260 hours a year, it will still take you 38.5 years to achieve 10,000 hours. It's not like any sport or other activity (music, writing, acting etc.) involving just your body or inanimate objects, where you can practice for hours and hours. Of course, if you have the time and money to have multiple horses, you cut down on those years, and if you've started as a kid, you might be ahead of the curve, but remember, only deliberate practice counts. Only perfect practice makes perfect. And only, if you've also got the baseline talent.
Depressed yet? Luckily, being around horses and making even small progress is incredibly rewarding, especially if you keep the big picture in mind. If you can stay focused on all the amazing things that happen at the barn, from the nicker when she hears your car coming, to that super feeling in the canter depart, and the moment he got the idea of taking a correct contact while still going forward... you can love your imperfect dressage riding self, and be the best friend and partner your horse could hope for.