Poor Tragedy

I don't know what to think about "The French lieutenant's woman" by John Fowles.
I have no idea. 
As a postmodern novel, it's brilliant. The book at 1st glance looks like a Victorian novel, even the language is Victorian, but it turns out that it was written in 1969. Each chapter has 1 or 2 epigraphs- excerpts, quotations from works by Darwin, Marx, Thomas Hardy, Jane Austen... and sometimes there are footnotes stating, explaining and discussing some facts of Victorian society in comparison to modern society, such as conflict between science and religion, politics, Darwinism, melancholia and hysteria, prostitution, codes of conduct, etc. This shows John Fowles's vast knowledge and profound understanding of Victorian England and comments on some fascinating differences between that period and today, the comparison we have to do ourselves in our minds when reading 19th century novels (1 of these facts, I should add, helps me understand 1 important point in Jane Austen's "Sense and sensibility"). Another wonderful thing about this novel is its metatextuality- in 1 chapter John Fowles appears and pushes the world of Charles and Sarah and Ernestina and all other characters away from the readers by saying directly that it's all fiction and everything's in his imagination (what does "the death of the author" (Roland Barthes) mean to an author writing fiction?), as the chapter's enclosed he goes back to behind the curtains only to appear again many chapters later, near the end of the book, this time sitting on the same train with Charles, 1 of the characters, gazing at him.
"The French lieutenant's woman" is also notable for his treatment of the story- it has 3 endings. Isn't this a mockery of Dickens's 2 endings for "Great expectations"? The 1st ending is a Victorian one, traditional, conventional. The 2nd ending is non-Victorian, but also a conventional ending. And the 3rd one is an unconventional ending and may be considered feminist. This novel looks like a Victorian novel but in some aspects makes fun of Victorian novels. 
And yet, I feel uncertain about it. There are times when I come across an important, influential, acclaimed novel which I know deserves its classic status, and yet I don't really like it. I can't explain why I don't like "Lord of the flies" (William Golding), for instance, it's rather personal. Now, with "The French lieutenant's woman", my view probably results from the fact that while I see its greatness as a postmodern novel, I'm not sure if it's really good as a novel, or at least I'm rather put off by the characterisation of the 2 main characters Charles Smithson and Sarah Woodruff, which is rather obscure, incomplete, even flat. I don't understand Sarah's personality and actions and don't see the motives- one may argue that she's not meant to be understood, the whole time it's repeatedly said by the narrator and many characters including Sarah herself that she's not understood and not meant to be but I'm puzzled from the beginning to the end as to what kind of person she is and why she does what she does, especially in the end, and I see it as a shortcoming rather than something deliberate because at the same time there's also something lacking in the characterisation of Charles, who to me appears quite shadowy albeit being the protagonist. I don't really know what kind of person Charles is, and at times, don't know why he does what he does, why he has certain reactions or says certain things.
I'm aware, not all novels focus on characters, their personalities, their inner lives, their thoughts... but in this case, for some reasons, I find it quite unsatisfying (which may be merely personal). 
In the end, having written this post in order to contemplate this novel further and to organise my chaotic thoughts, I still feel unsure.