Hi! I have a pony who has the same background as your mare. He was lightly started as a 2 or 3 year old then sat until he was 7 and now I have him. When I started riding him, he did the same thing as your mare. He would walk, but no matter how hard I’d kick or even if I used a crop, he wouldn’t stay trotting (or sometimes even walking) for more than a few steps.
My trainer told me that this is EXTREMELY common with super green horses. Does your mare respond to verbal cues when you lunge her? If not, I would start there. If she does lunge well, then it’s a riding issue and what my trainer had me do was get on and have a goal to walk one lap and trot one lap in each direction. That’s it. At first, pony would stop. Trainer had me pull him in a circle (bring one rein all the way to your hip if you have to) and get him to simply move his feet. That’s all you’re looking for at this point, her to move her feet when you ask her to. No kicking necessary, just pull her into a circle and as soon as she moves her feet, let her keep going forward in whatever direction she’s heading. Steering is not the objective right now.
The first ride doing this, I think he planted his feet 5-6 times and I had to pull him in a circle to get him going again. Next ride, he only stopped once. The third ride, no stops at all. Now that he’s not stopping unless I ask, I can start to add laps and extend our rides a little. They figure it out quickly, but you have to make sure you’re in the right mindset and have all of the time and patience in the world to work through this.
Oh, another important tip…once she’s moving forward only let her stop when YOU ask. If she starts to slow down or you feel her thinking about stopping, keep her going another step or two and then give a firm WHOA cue. Stopping is up to you, not her.
Let me know how it goes or if you have any other questions! Good Luck!!
