I'm used to riding a pony, but now I'm riding a horse with a very very big, swinging stride. It was pretty awkward riding her for the first time. Is there anything I can do so I don't look like a flailing fish out of water? It probably wasn't THAT awful, but it felt like it. Even walking and sitting the trot were pretty hard. Not really walking, it was just strange having this big movement. But mostly posting was...not pretty.
Time in the saddle.
I know...not a quick fix but the more you ride the faster you will adjust.
Riding on a longe will help too, so you can work on getting used to the new movement and your seat without worrying about your hands/reins/steering.
The more you ride, the faster you'll get acclimated to the new horse's strides. In time if you ride a pony again you'll laugh at feeling like you're trying to post on a jackhammer.
And look on the bright side...the more varied horses you ride...the better you get overall. It might not feel like it as you get used to all sorts of different strides, personalities and attitudes...but it does improve the rider.
You jump in the saddle,
Hold onto the bridle!
Jump in the line!
do some core muscle exercises, you will need to keep a strong and stable core to deal with the bigger stride. A bigger stride creates more thrust from the horse's hind end, a stable core helps you to stay "in control" of your posting so you don't look like you are flopping around.
Every man has a right to his opinion, but no man has a right to be wrong in his facts. Bernard M. Baruch