Forlorn help with a book....

[Fartooold]

I read a book 40 years ago (!) And have tried for so long to find it again without success, I'm hoping the power of MN can help!
It's Arthurian , but, as I remember it, Arthur and Guinevere are childhood friends, Avalon is a make believe island they play on, they agree to a marriage of convenience, Arthur knowing thatvl Guinevere is in live with Lancelot. Merlin is nit a magician, but is an holistic wise man, teaching the kids to breathe through reeds underwater etc.,

It was a BRILLIANT book ( in my memory), but what the buggery was it????

Help 😁

Could it be The Child Queen (The Tale of Guinevere and King Arthur)?

That's so close it might be..... thank you!

In my mind though, it was more the relationship between Arthur and his tutor Merlin that dominated the book. But that might be a false memory😁
I'm pretty sure it followed through and Arthur had to send Guinevere to a nunnery to preserve his reputation - with her tearful agreement.....