I'm sorry for your pain. There are many women on this forum who are in the same boat. For what it is worth I don't think you have found out "why" - the why can be a very complicated thing.
Tell us more about your Marriage in general? What else have you read? What have you tried? Have you been to any counseling? Do you suspect infidelity?
There are many wise people here who can help. There have been some successes on this board. Do not give up hope.