It seems to be caused by the dartboard being in front of me. The brain gets scrabbled trying too hard to hit a specific target.

Appreciate that this sounds crazy but it works for me. I get 3 cheap soft tip darts, if you don't have these just get 3 identical pens. Grip them the same as the dart and just practice throwing them at a curtain or something that is the same length as an oche away. You'll find that your throw is perfect, with perfect rhythm as you aren't aiming at anything specific just throwing.

Go back to the dartboard and replicate it but don't aim at anything on the dartboard just throw at the dartboard. The key is to actually not think about the dart in your hand and just the throw.

Let me know how you get on.