Ending a romantic relationship with a co-worker can be especially complicated because you will have to interact with the person after your break-up. Since it may not be easy to avoid seeing the other person it's especially important to take the time to plan your break-up thoughtfully. The most desirable outcome would be that you can both continue to have successful personal lives despite your affair and the fact that it has ended.
Choose a time and place away from the office to meet with your partner. If you have been having a long term affair with your partner there may be serious feelings involved. It is helpful if you can meet and discuss your break-up when you will both have time away from the office. You may want to time it when a long weekend or holiday will give you extra time away from each other so you can return to your workplace with a fresh outlook.
Be direct and firm about your desire to end the affair. If you're not firm and clear. your partner may not see that this break is final. Since you work together you will not have the ability to avoid seeing him, so it's important to be very final about your decision, and also communicate it in a thoughtful and caring way. It's important to have honest closure so you can both feel comfortable moving on with your professional obligations in your workplace.
Develop a plan for how you can both cope in your shared work space. It's a good idea if you can agree to give each other space for at least three to six months, and this may require planning to use different areas of an office, changing lunch or break routines, and generally ensuring that your break-up doesn't affect your professional performance. Break-ups can be difficult, and your don't want yours to undermine your credibility in your work. Allowing anger or resentment from an intimate relationship to affect your professional performance can do just that. Avoid gossiping about your partner, and maintain a sense of civility on the job.