Most of the people I know who work out seriously do so because they have such an amazing outlook on life. To be who I want to be I'm going to work out to be more positive more active. It's proactive.
You've just got to have a sense of respect for the person you have children with. Anger doesn't help anybody. Ultimately you have to say forgiveness is important and honoring what you had together is important. But it's easy to say and harder to do.