I think this was cross-posted into the H/J board - fully agree with @Scribbler!
Here’s what I posted over on the dark side (#hunter-jumper ) :
To me this seems like a weakness issue - Particularly core strength. I’ve struggled with this after taking time off from riding after an injury and then after having kids.
Practice finding your balance at a half seat, two point and three point at a walk.
Focus on finding your center of gravity without relying on the horse’s movement to thrust you into that position, and work on getting yourself into those positions from a full seat.
Once you’re able to reliably hold that at the walk without grabbing mane or leaning on their neck, drop your stirrups. After that’s been mastered at the walk, start it all over again from the trot. Rinse & repeat at the canter and adding in ground poles, then jumps.
Anytime I’m feeling insecure over fences (or during more complex “questions”), I like to go back to basics to really diagnose the root cause of the issue. 9/10 it comes down to fitness level. I supplement my riding with a stationary bike and rower - we rely on our core to be an effective rider more than is recognized. Bonus points for easing back pain…!
Take your time and be patient with yourself - good luck!