need help with the title

Hello!

There is a book telling a story about a man who was in love with a woman.
Yet he married another one. But for fourty years or so, he was writing love
letters to her, but didn't send them, thinking she's forgotten about him by
now.
Then, when he was much older (after 40 years, I guess), his wife died. And
the man left to see wheter his "mistress" was still alive. He found her, but
I think she just died a few days earlier. He also found out, that she was
desperately in love with him, too and that she's been waiting for him for
forty years. Realizing his life's has been a "mistake" he commits suicide.

I think the story goes on in Russia. I found the reference to this book in
"The Unbearable Lightness of Being" by M. Kundera, I think

So, can anyone help me with the title?
Does such a book even exist?

Thanx a bunch!

Jurca