Well first off, congratulations. You're going to have to tell us how it happened.
From what I hear, it is a rocky road for both partners, no matter how well meaning they are, because we're human. So, it's normal. I just read an excellent book you should get a copy or read it at the library, and it covers a lot of how you both feel and what is happening and how to work through it. "After The Affair" by Janis Abrahms Spring, PhD.
