I played harmonica since I was around 10 years old so when I got an Erica I took too it quite well.
The main problem I had was that I used to breath in and out as you would with harmonicas. Not a great idea with quicker tunes as you tend to hyperventilate....
Just the same with me after years of harmonica in my teens. The problem has long gone with melodeon (started some 24 years ago), but is very much a problem playing Irish music on Anglo concertina even after more than a year. However there are encouraging signs that he problem is at last beginning to disappear.
Firm attempts at relaxation while playing, plus practise have to be the answer.
Posture is important too and it made a big difference when I really concentrated on sitting upright rather than just sitting hunched over the instrument peering at the dots. Arm and shoulder stretching prior to playing and a few deep breaths also help, especially if like me you have a tendency to tense up.
'best, Rob