When you love someone, when you really love someone, you don't want to believe that person could do you wrong--not without having their hand forced by another. It's easy to blame the "other woman" (or the "other man" or what-have-you)
But that's not helpful; not for her, not for him, and certainly not for you. This woman doesn't need to be your best friend, but you shouldn't hate her. Need further convincing? Read on.
1. She didn't cheat on you.
This woman had no obligation to be loyal to you. She wasn't the one dating you.
2. She only followed the lead of your S.O.
By giving into temptation, your S.O. showed this woman that his relationship with you isn't worth respecting. She took him for his word--a mistake you both made.
3. It's likely you were both hurt by the same person.
It's easy to look at the other woman and let yourself believe that you're nothing like her and that you have nothing in common with her. But the other woman is very rarely some sort of villainess, or nymphomaniac that gets with any guy she wants to regardless of whether or not he has a girlfriend.
It's probable that either she had feelings for your guy too, or that she didn't know the state of your relationship with him. Either way, consider whether or not she too feels hurt by what happened.
4. You shouldn't hate her for providing "competition."
There shouldn't be competition. Your S.O. should be yours, and you shouldn't feel that any woman can somehow steal your man away from you. He isn't a playground toy. End of conversation.
5. Guys CAN control themselves. And so can girls.
There's no amount of sexual tension in the world that forces someone to be with someone else. Everyone can always say no. Don't give your S.O. a free pass just because they say that they were overcome with lust or some bullshit like that.
6. You're too smart to fall victim to mindlessly hating the other woman.
Some part of you knows that she isn't the one to blame here. Some part of you knows that the person you're in love with is the ONLY person who really wronged you, whether or not you're willing to accept it.
7. She did you a favor.
If your S.O. can be that easily stolen from you or convinced to betray you, it's likely that person isn't worth holding onto.
Bonus Point
You shouldn't blame yourself either. There isn't a person in the world who is perfect enough to force their S.O. to be faithful.
Everyone must choose to be faithful to themselves. And just because one person didn't have the morality and willpower to do so does NOT mean that you weren't worthy of being respected and cherished. You are more than worthy; don't let anyone ever make you feel otherwise.