I wish I could remember how to be happy again.
Happiness is created by progress. Instead of trying to remember how to be happy, heading off in a direction of your choosing will make happiness show up; it's a proactive thing that puts you in control. It may just be a moving away from him initially, tough to do when you work together I realize, but then it can become a moving towards something, the life of your dreams for example. And as a bonus, once you commit to that, he will just fade into the past.