A horse can't do a half pass on a longe line, sorry. And a horse that's being longed in a halter or loose reins on his neck can't do an extended trot. An extended trot is not just a long or fast stride. That's an active trot, but it's not extended. That comes from a long, long time of training and development. It is the result of 'recycling' all that energy with half halts, causing engagement and a swinging back - further, it would NOT be something that would be safe or healthy to try and do on a longe line (even if one did know how to push and half halt on the longe line while longeing the horse) or even on a circle - not for a very, very long time....but you'll learn that...and more.
No, it's not completely true that any horse that can walk trot canter can 'do dressage'. Of course any sound, healthy, pain free horse can be schooled in dressage at home, and provide fun and pleasure for his rider. And of course, as often said, 'dressage means training'. But it doesn't really mean just 'training' to go and compete (or to meet traditional ideals in dressage at home), or to excel in competition at the top levels.
Too, most people in dressage never ride at the top levels (FEI). Most never go to the Olympics, or a national championship - most never go above training level, to be honest (99%). But it's within everyone's ability to ride to fourth level - if they put in the work and keep their horse sound. The really hard part is sticking with the instruction and doing the work for years and years. No one really learns correctly and goes up more than one level a year, and even that rate of progress will only be possible with frequent lessons with a VERY experienced trainer.
Your horse is beautiful and elegant. You sit very well, with a nice straight back and correct legs - for hunt seat, though, not dressage. You'll need to learn dressage position and how to use reins, seat, leg. For most people, there are two things that limit them in dressage:
1. keeping their horse sound (selecting a horse with 'sound conformation', preventative care, and treating any lameness that occurs)
2. Learning it themselves, correcting their position, learning the figures, exercises, and accepting instruction
It is only at a very, very high level that the innate talent of the horse starts to be a factor in competition success. People worry far, far too much about the 'quality' or 'potential' of their horse at early stages, to be very, very honest...a little reality check - it is very, very unfair to expect most horses to 'do it all' in dressage - allow the rider to learn and teach his own horse, excel at every level. That's often a young person's dream, to ride their beloved horse that they have right now, in the Olympics! It just doesn't really happen that way. The kind, loving horse that allows the rider to learn on him is worth his weight in gold, but it usually takes a very different horse to excel in competition.
But whether the horse will be 'successful' or not depends on how you define 'success'. Does it mean having some fun at a local 'schooling' (club) show, or winning a national or international championship, or something in between the two? Does it mean a local ribbon at training level, or Olympic success at Grand Prix(the top level)?
Even if success doesn't include competition at all, learning dressage well really requires guidance - from a knowledgeable, preferably USDF certified, instructor, and frequently. An occasional lesson with this instructor and then that one, even occasional clinics here and there with this and that famous big name rider, doesn't lead to much success - neither does working with someone with no real experience or training in dressage. When I was moving to a new area a hunt seat trainer came up to me and offered to get my horse ready for a show - just slap on those draw reins and he'll be perfect! Nothing could be MORE wrong. There's no point in wasting time with poor quality instruction (and someone who rides in another division, sorry, they just do not know dressage - the instructor needs training and experience or he just can't teach dressage).
Working alone too much, it's just too easy to get into bad habits, and reading a book or seeing a video just doesn't teach that 'feel' one needs - in fact, 'book only' dressage riders who don't get regular help are usually the most far off the track, they think they're doing it right, but they aren't. They can quote tons of theory but can't ride or train correctly! Books and videos help, but they need to be just one part of a total program, and they should never cause a rider to stop his lesson to argue with his instructor!!
Most riders who work alone alot, don't realize they're leaning off to one side, hiking up one hip or applying the wrong leg - keep in mind even the best Olympians have coaches, and usually work with them every single day - even after winning Olympic or world championship medals!
At the same time, it doesn't work at all to be too perfectionist or hard on the horse or oneself - these people get bogged down and never get anywhere.
But it doesn't work to spend most of one's time walking on a loose rein, trail riding or doing other things with the horse. Most successful dressage horses are trained 5-6 days a week - in dressage, and nothing else, with hacks after the lesson as a reward. They are very, very happy doing so - IF the training is correct, they do not get sour at all....the rider is more of a problem! Horses really like a routine - it's more often the rider that doesn't want to stick with it every day!
Lessons long term, frequently are a big investment in money, but even more so, in time - both during the lesson and in practice after. A person will spend months, years, getting lessons - mostly to 'unlearn' what they thought dressage is! Too, that practice time is important. You won't find anyone moving up the ladder in dressage on 1, 2, 3 brief rides a week. Too many people think that because there's no jumping, dressage must be easy. It is not easy to do well, or to learn. And it really takes time, because much of it is building the muscles - in horse and rider.
It's true that many people think forcing a horse's head down with rhythmic jerks of the reins (it doesn't actually matter if it's jerks, 'spongeing', 'working the fingers' or even very, very gentle, it's still wrong), some appliance like a gogue or draw reins, means 'dressage' or 'on the bit'. Jerking back and forth on the reins, even gently, to get the horse to put his head down, is not going to lead to any success.
But many people go totally in the opposite direction, declaring that we should do dressage without ever using the reins at all, since that mean old bit is so painful and cruel, and contact with the reins is so cruel! But look at the Spanish Riding School - the horses are ridden with a frank, even firm, contact with the reins. While jerking and pulling or draw reins aren't the answer, the reins can't be tossed away either. What's needed is something much more difficult than either of these - to use the reins in coordinated, sympathetic, educated way with the seat and leg aids. The skill of the rider is what makes riding on a contact pleasant and rewarding for the horse. And that means instruction.
