I don't know why she would experience a 'sudden' change like that after her layoff. It happens sometimes, but I've never quite understood why.
I have found that bending them, sort of walking on a 45 degree angle with their butt pushed to the side, and letting them straighten out only when they stop jigging, works. Does that make sense? So if the horse starts jigging, for example take the head slightly to the right, and put your right leg on to push the butt to the right. I don't know why, but it has always worked for me.
I am going to tell you what my trainer taught me that worked for one of my mares that I had problems with trail riding. And I don't know if this will work for yours, I never met her and I've never met you, but it worked amazingly for me.
This mare I'm talking about is super brave and very willing, but just super sensitive and with a little bit of 'go' when she gets anxious.
I always wanted to give her more freedom, give her more rein when she got anxious. This was the wrong thing to do with her. She wanted to know someone else is in charge at all times, and I found that the easiest thing to do in dealing with her anxious behaviour is to not let her get into that state of mind at all.
You want to take the reins pretty short and really even, keep the head up and pointing straight (or wherever you want it to turn), and keep your hands around the crest of the neck. You want to make sure you are so square and tall and strong, so nothing can move you. Even if the horse did a 180, you'd still be in that tall, square position right over its centre. Basically, you want to give yourself as much control as you can.
I'm a western rider, and at first I was resistant to this way of riding. I wanted the horse to go along with its head down on a loose rein, and be able to neck rein...all that stuff. I was told that's fine, but when your horse is being an idiot, if it's actually wanting to buck and bolt like yours is, control is way more important than looking quiet and westerny.
And you basically want to ride in a confident, strong manner. You probably won't want to gallop around out of control, naturally, but picking up a good strong trot, weaving, doing circles, even cantering if you're comfortable that (s)he's not going to buck or bolt...give the horse work to do, something to think about, something that proves that you are in control.
My horse improved by leaps and bounds (and so did I) when we both knew who was in control, and that the person in control was confident. Again, I can't say for sure if this will work for you, but it did for me, and it's worth a try.
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