I rode casually growing up – I always wanted to jump but didn’t/couldn’t ride consistently enough to get good enough to jump!
In my early 40s I got my first horse and settled into a h/j barn. I did the Rusty Stirrup division at my first few shows and eventually moved up to 3’ - 3’3".
What I’ve found is that the riding I did as a kid (mostly bombing around bareback) has helped tremendously in terms of balance and lack of fear. But the thing that I really struggle with is having an eye for that elusive “distance”. Most of the people I now ride with rode pretty seriously as kids (h/j) and don’t have an a problem with it at all. Everyone insists that I will “get it” – I’m so ready at this point!!
In terms of your hands and stiffness – try having some lunge lessons. That will take your hands out of the equation and allow your legs and seat to develop independently. Another thing you can do, just as part of your warm-up, is bridge the reins in one hand and do a variety or limbering/stretching exercises with the other – hand on hip, behind the back, out to the side, out in front, over head, forward circles, backward circles, etc.