Wartime/historical romance novel about the life of a young woman/ fashion designer.

Please bare with me; it's a long post, but I want to make sure I cover everything. It is a fiction book and it is about a girl during a war (WW1 or WW2). Key plot points include;

1. The girls mother or other family member turns out to be a famous fashion designer/ dress maker. The girl is also a fashion designer 1.5) I also think that as a kid her mother and possibly sister died in a bomb explosion.
2. The girls family becomes trapped in flood waters at their house on the cliff- someone or some people die
3. Her and her husband are involved in a car crash on a rainy night after an Opera or theater production. They were fighting beforehand, possibly because she saw another guy whom she also loved. 4)(I think) As a young girl she fell in love with a boy when they lived in a refugee (pretty sure it was some type of cave) during the war- they made a promise to marry each other when they got older.

The girl grows up and that's when she learns that one of her family members is a famous fashion designer/ dress maker.

The guy that promised to marry the girl when they were younger didn't see the girl again until they were much older.

The other guy (husband) becomes jealous of the other guy, that's how they were in a car crash. I think the guy dies or is seriously injured.

I'm pretty sure that the girl's grandmother dies in the flood waters.

It's a historical romance novel and the copy I had was a paperback; it had a blue cover and I think there was a lighthouse or cottage on a cliff.

It's a ridiculously long book; like more than 2000 pages, it's also pretty old. Probably around the 40's-70's.The text is also reasonably small. I read it when I was a kid; around the age of 8-13.