Just because you may be the greatest of friends living separately doesn't mean it's a good idea to become roommates.
If you're not a happy person, fake it. Cheerfulness is contagious, and if your fake happiness can rub off on your friends around you, eventually that happiness will rub back onto you and you'll not need to fake it anymore.
If a relationship ends it's not because one person failed the other. It ended because the two people were just simply not meant to be.