@CSalts
I don’t think I’m a bad parent. All I want and have ever wanted is for DD to be happy, she is my priority.
I don't think anyone has insinuated that you are - they've just suggested that if DD IS your priority, you should have proffered a more supportive view rather than telling her that your entire plans to continue playing Happy Families will be annihilated by her decision to leave her DH.
My concern is whether ending her long term relationship and marriage is really the answer.
Why is this your concern? If your DD, your priority, has decided she has fallen out of love with her DH and that they've grown apart / no longer got things in common, why does your concern about her doing what she wants to do, trump her feelings of concern about a dead in the water marriage?
Often you don’t know what you have until it’s gone.
Yes I agree, and you'll really miss DD (your priority) when she realises that you don't have her best interests at heart, and instead want to keep up appearances amongst your social circle.
The family can all see that what DD and SIL have is very special.
I have had to sit on my hands for this comment. You know absolutely NOTHING about what goes on behind closed doors. Believe me, I've been through the mill with my "oh so charming" XH. Behind closed doors he was a monster and my family never checked in, and they never knew. For a decade.
She is risking throwing it all away for excitement or a fairytale life which in reality doesn’t exist.
How do you know? Just because you are basing her marriage and her future on your own experience doesn't make you the expert does it?
I don’t think anyone can honestly say they are happy in their marriage 100% of the time.
Projection much? Have you ever wanted to leave your marriage but couldn't / wouldn't / shouldn't because of outside pressures from your family / wider family? The feeling of letting down The Institution? Becoming a statistic?
I hope you've backtracked massively and already phoned / revisited and now given your daughter the support she needs.