What's the best/easiest way to fix this? I'm pretty sure it's causing me spin out problems. Thanks for any help with my limited DIY skills!

mix superglue and graphite (grind a pencil), wait until dry then wet sand.
For spinout try and move mast base more forward, harness lines further to the back.

Thanks for the comments. Although I've been windsurfing for decades, I've mainly been into wavesailing and only recently rediscovered how awesome powered up blasting can be with modern freeride kit and a big sail - Blast 117 plus 6.7m / 7.7m Speedster. So Imax1/powersloshing yes it could well be a technique thing but I've never had a fin release quite as often as this one. Not to mention the weird whistling noise the fin makes above a certain speed!
Will have a go at filling it in and sanding if only to stop me thinking about it!!

I don't think it's big enough to affect your sailing at all. However you can fill it with super glue and talc or baking powder. Only use graphite if you want it to look like carbon.