It probably is just because you need to get back in shape and used to the whole deal. Do a lot of no stirrup, standing at the trot and canter, 2-point at the trot and canter, dropping one sirrup or the other, and trot fence excersizes to help you. Also, maybe you should try perfecting the lower ones? Always think of your shoulders, elbows, hands, heels, and legs.
I remember when I was first jumping the bigger ones and I did some of the same things your doing, my trainer put a crop behind my upper arms and made me jump gymnastic lines. It worked for me, but if your on a horse that needs a big release, I wouldn't recommend it. Also try relaxing some too(:
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