<human>: ME:

I'm currently engaged. A few months ago, I was told by her ex that my fiancee (girlfriend at the time) was cheating on me with him. At first I didn't believe him, because they dated for a whopping three weeks before she called it off on account of him being extremely weird and rude, and she had never maintained contact with him after that. However, he showed me her FaceBook account and the messages they had been sending back and forth ("I love you" "The sex was great" etc.). Obviously, I was pissed. So I confronted her, and her response was "I don't have a FaceBook." I didn't believe this, as I had just seen it, and pulled out screenshots of the messages to show her. She claimed she had never seen them in her life and swore she didn't have an account. So after some arguing I ended up confronting the ex and asking to see it again, playing along like I wanted grounds to break up. While on his phone looking at it, I realized he was signed in as her. The email address to "her" account was also his, and apart from the messages, the account had almost zero activity. He literally created her an account and used screenshots of pictures from her Instagram to make it look legit with the intent of breaking us up.

I normally try to play it cool in these situations, but I lost it. I just punched him in the face and slammed his phone out of his hand and left. He hasn't tried interfering with us again, but it still is absolutely sad that he went to that length just because a girl he dated for three weeks almost three years prior had a serious relationship.

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Provide a brief overview of the text.
<bot>: three-week ex of my fiancee from three years ago made a fake FaceBook account of her to try and break us up because he didn't want her taken in a serious relationship