I find that with everything else being equal, the height of the strings is important. My Serenade was a little lower than my teacher’s Prelude, and I place it on a box about 4″ high when I practice, then I have no problem.

You might want to try breathing when you’re not focused on

Wow, I read this before, but just recently have had the same symptoms
crop up. I find that when I am reaching for the lowest strings
(something that I am doing a LOT lately since my harp has more strings
than anyone elses in our ensemble) I catch myself hunching over,
leaning forward and holding the harp against me with my right hand. If
I don’t hold on, I will push the harp away from me with my chest. I
have very short arms and don’t have a problem reaching for the
occasional low C, but in one piece we are doing all my notes are in my
lowest octave. I made a comment once that I didn’t know when I got a
bigger harp I would end up playing string bass! I have been trying to
sit a bit higher than I am used to doing (short arms, short legs) and
that helps some. I also find that whenever I take a break, it helps to
do shoulder rolls.

Leigh