I went through this many times :frown1:, so I know exacly how you are feeling. By many times, I mean that at 7 yrs my ex decided that she was tired of being a wife and a mother. She complained that she had "never been able to live her life". She left us for about 2 months and then decided to come back. I spent the next 5 years trying to be the best husband I could be. Everytime she wanted to leave, I would ask her to go to marriage counseling, which would only last a couple months.
It was hell until I finally realized that I was fighting a losing battle and filed for divorce.
It's so hard to tell when you should be trying to hang on and when it's time to let go. Once you decide to let go, you can work on rebuilding your life and be the best Dad that you can possibly be. You are lucky that she has agreed to not fight over custody. That makes it twice as hard.
Keep coming here for support (unfortunately, there are more low times coming). I wish I had the friends that I have here at the time I was going through mine.