A partner is supposed to love you, and just you. When you break up and you find out your partner was seeing someone else, you will have an array of emotions to deal with, from sadness, to loss, to pure unadulterated anger. Being angry will show your ex how badly you were hurt, but if the anger remains with you long after the breakup, it could poison your future happiness. Letting go of the feeling is a key part of reclaiming your life and your emotional security.
Distract yourself from thinking about your ex partner and the person who took him or her away from you by going to the gym. An April 2009 review of studies on the emotional effects of exercise published in the Internet Journal of Allied Health Sciences and Practice found that moderate aerobic exercise helps boost mood significantly. Other beneficial exercises are yoga and tai chi. Burn off your rage in the gym. The added benefit will be the sweet revenge of a trimmer body for the next time you meet your ex-partner.
Identify the aspects of the breakup that made you the angriest. These could include sexual betrayal, emotional betrayal, the loss of trust or the loss of a friendship. You may find that some of your anger derives from feeling hurt that somehow your ex-partner's new love seems more attractive or nicer than you. Identify the source of your anger; then you can begin to deal with it. Use this knowledge to figure out what you want from future relationships.
Take a good hard look at your own behavior. If you can assure yourself that you were a good partner, ask yourself how you ended up with someone who repaid your trust and hard work by cheating on you. Ask yourself if your ex lived up to the basic standards you expect from a relationship. Then you can start letting go, and the anger should lessen as your ex-partner becomes less important to you.
Take a look at the situation from the points of view of both your ex-partner and their new love. Ask yourself, honestly: If you were in their position, would you have done the same? If you met the love of your life when you were already in a relationship, would you risk hurting your current partner? If you can say with certainty that you would never do such a thing, that tells you your ex did not live up to the standards you set. Take this knowledge with you as you seek a new partner.
Go out and meet new people. Don't talk about your feelings for your ex on a date. Being "in the moment" and concentrating on your date will stop you from talking about your anger. You will soon learn that there are other people out there who can make you happy.