+1 for just letting the bike go and getting a bit back. Hitting roots at right angles helps a lot too.
It's the same sort of technique / riding position as you use for riding on sand & loose rocks. You just need to let the bike do what it wants and make the merest suggestions on the steering. The closest off bike situation I can compare it to is aquaplaning in a car.
Another option for upping your skills would be to nip out for a ride on the wrong tyres. I rode around on semi-slicks in the mud for a while when I first got back into bikes, and whilst I was slithering all over the shop, I got a lot better at catching a sliding bike very quickly. 😆
Maybe that would get you over the fear? If you know you've got a decent chance of catching it, you won't worry so much about sliding, won't tense up so much, and therefore won't slide so much…