Commiting infidelity is probably one of the worst things a person can do when in a serious, committed relationship. The consequences are often far reaching, and it might take nothing short of a miracle to get your relationship back on track. Telling your partner you are sorry might not feel like enough, but it is a positive step in the right direction.
Consider what you are going to say to your partner before you see her. Think about your apology, and make it genuine and heartfelt. Do not try to justify your actions, but do accept responsibility for your mistake. Ask yourself why you cheated in the first place.
Do not offer excessive compliments or be insincere. Your partner likely feels inadequate, and might not believe you. Be prepared to answer awkward and difficult questions. Be honest and maintain eye contact when talking to her. Make your apology in a private setting.
Be open and transparent with your partner; this is particularly necessary if the relationship continues. Realize that your partner might not trust you for a very long time. Do not expect her to accept your apology straight away.
Show your partner how sorry you are via actions. Avoid buying cliche items such as flowers and chocolates. Spend quality time with your partner, reassuring her that you are committed to the relationship. Do things together as a couple. Put more time into the relationship.