Call in sick. Emotional injuries take time to heal just like sprained muscles, shattered bones, viruses and infections do. If your broken heart feels unbearable, give yourself a day or two get over the worst of it.
Move slow. Focus on one little task at a time. Move forward on projects in baby steps. Don't take on extra work. Chances are your mind isn't as sharp, nor do you feel particularly motivated, so be easy on yourself until your energy level returns closer to normal.
Schedule mourning breaks. People take smoke breaks and coffee breaks and even gossip breaks, so why not give your broken heart scheduled time to mourn each day? Walk around the block, sneak to a café across the street, shut the door to your office. Cry, cry, cry, cry! Then get back to work. Should you need it, you can take another 15-minute mourning break in a few hours.
Keep the gory details out of the office. Your breakup may be the only thing on your mind, but talking about it with your coworkers will make you feel worse. Don't debase your emotional state by offering up your breakup as fodder for office gossip. If one of your coworkers is a friend you want to confide in, do it outside of office time.
If you must confide in your boss or coworkers, do so in vague terms. Say something like, "I'm sorry if I seem not myself lately. I'm going through a tough time and I'm doing my best to keep on top of my work. I should be back to normal in a few weeks." If people inquire more deeply, say politely, "I appreciate your concern, but it's easier for me to not get into the details. Thanks for your help."
Save the sudden moves for later. Your boss micro manages. Your work numbs your mind. Your coworkers stress you out. You want to leave this rat race and try something new, anything new. While I'm the first person to support a reevaluation of life and a change of direction, keep in mind that anything that felt uncomfortable before your breakup is bound to feel 100 times worse. Save major changes in your career until after you've had ample time to mourn your loss to ensure that the moves you make are based more on clarity and less on foggy emotional impulses.
Be grateful for the routine. A steady rhythm helps tremendously to keep you grounded as you grieve. Once you heal, big dreams will emerge again, your energy level will pick up again, love will fill your heart again. In the meantime, be thankful to have a job that provides you with food, money and healing.