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If you have ever been cheated on you know exactly how it feels. You feel hurt and betrayed. Sometimes even feeling stupid because you didn't see the signs.
But don't worry, you're not alone. There are so many people out there who know this feeling. Confide in your friends. Talking about it will help.
So what next? Once you find out you have been cheated on and it's all out in the open. My suggestion is to find out why it happened. Perhaps too many drinks with the boys? Or he just wanted someone new. Whatever the reason is that he/she cheated you need to decide wether or not you take them back. Knowing why they did it sometimes helps. I was once cheated on and it was because he liked my friend all along and was using me to get to her. If after he cheated she didn't want him and he tried to get me back, knowing this information, I would have said no.
When it comes to being a cheater, I have never done it, but I don't know how someone can. Knowingly I mean. How can you knowingly do it to your partner? Or with someone you know has a partner? No one pats you on the back and says good job mate you screwed that married chick you met at the pub on Friday. So what do they get from it? Satisfaction? But how can one really be satisfied if they have to keep all the emotions inside?
This I'll probably never know.