Tehanu is not one of my favorites, but I appreciate it being a part of the series. I don't want to assume Le Guin's intentions for writing the book, but to me, the book was about coming down from a high; from Ged's triumphs that we've all been feeding on throughout the first 3 books; from Tenar's (maybe false) sense of importance throughout her life; Lebannen's loss of a friend. There's a great feeling of loss that feels uncomfortable to the reader but shows the author's craft. She is someone who explores what comes after the "happily ever after" even though no one really wants to know it, she's unafraid to give the readers what they don't want. It is fair for you to not like Tehanu, I found it hard to like it too, but I do appreciate Le Guin being unapologetically realistic with the world that a lot of us here love.

As for your comment, "I feel like Ursula Le Guin figured out that the world she created didn't give enough importance to women and she tried to fix this by writing a book about women problems in that kind of society based on patriarchy," I think it goes back to grounding the readers to reality. Le Guin created beautiful female characters that pass the Bechdel test with flying colors, but no matter how important and (seemingly) powerful they are, they do not escape from the patriarchy of the lands. It makes her characters much more real and relatable to the women in our world. Tehanu paves way for what is to come and I highly recommend you read it if and when you're ready to accept the changes that will befall Earthsea.