It is not advisable to take matters into your own hands. You might do something and then regret later. What you should do is calm down first and think things clearly and then you will know the best action to take. Once you are calm, you can confront the situation.
Confront your partner. They are the one cheating you. As much as it hurts and as angry as you probably are, the other party is not the one cheating on you, your partner is. Good advice given about calming down first before you talk to your partner. It's a tough thing to get over but you don't want to say or do anything when you are really upset no matter how justified it seems. Good luck.