welcome Rob!!!
good luck with teaching windsurf to your fiancee!! this usually leads to some heated arguments on the beach!!!
It is really hard to teach someone that you are emotionally attached to.
It is a mix between
- she doesn't want to fail as she then feel like she's the worst ever and you would probably prefer to have another fiancee that is a windsurf champion already.

- you (deep down) want her to progress faster than anyone else

- she thinks you are too impatient, directive..and she can't hear what you are saying anyway, or that you are shouting and she could hear you

(no need to shout!!

)
- you are maybe not as tolerant when she doesn't understand or succeed

Best thing:
few classes with a windsurfing instructor, or change between time when you teach her, time when you leave her on her own (when it is safe obviously) and time when maybe a mate of yours teach her ( and be ready for the : "he's a lot nicer than you when he teach me"

)
seeya on the water!!!