a) Moving harness lines further back. (Sounds daft but try it)
b) Make sure rig is correctly set. (ie downhaul etc) (And dont sail overpowered)
c) Keep rig forward but sheeted.
d) Fin is easily big enough for that size sail (perhaps a bit too big) but check for dings/ scratches and fit in board.
e) Dont load fin (or push upwind) until well upto speed.
F) Try and catcjh spin out on its approach. Push rig forward.
We all spin out on occasions (lots of them sometimes) especioally I find in choppy water where air gets under board. Stick with it but experiment especially with technique for recovery. (little hop and pull board under you whilst bearing rig away can save it. Dont just let it happen and become prolonged. Try and react immediately.
(Years ago if we had a board that span out loads (as some did) a forefin would nearly always cure it. Not available now but borrow another fin and see if it feels same.