Focus on your job and hope that things don't get weird. This is one of the many complications that comes with dating the boss. If you want to keep your job, the best thing you can do is to keep your head down and be as friendly and professional as possible. You've already mixed business with pleasure in the most potentially career damaging way. Now all you can do is hope that he doesn't decide that having you around is awkward and find a reason to give you your two week notice.

"I am too busy with work" is usually guyspeak for, "I want out of this relationship but don't have the cojones to fess up like a man." If he wanted to be with you, he'd make it work. Maybe the relationship was starting to affect his job, and he chose job stability over you. Perhaps his coworkers were noticing and it was getting awkward. Or maybe he's just done with the relationship. It sucks, for sure. Even worse, you now have to work under your ex and hope that he doesn't turn on you.

Hopefully things ended on good terms and you can both separate your personal and professional lives. At least he didn't cheat on you, or start dating someone else in the office. Right now the only thing you can do is stay out of his way and keep your personal life out of the office. Try to have a life outside of work so you don't obsess over it. Go back to treating your boss as, well, your boss. Don't bring up your relationship at work with him or with coworkers. Stop emailing him, IMing throughout the day, whatever you were doing before. Don't ask him what he's doing after work, or why he broke up with you. You shouldn't expect special treatment, and you shouldn't let him treat you like garbage in the office just because you're no longer sleeping together. Start to move on. If things get too uncomfortable, consider looking for another job. Seeing your ex every day isn't easy. But if you're professional around him moving forward, you'll come out looking like the better person.