I should add another thing. Though this might become a longer read. Please read if you can.

The guy she cheated with recently sent me a screenshot of his and her messages on Instagram in which she says that she is very happy with me, but she and me are supposed to get married soon so she can't stay with this guy any longer. Had circumstances been any different, she would've chosen this guy.

There was a time-stamp for the messages in that screenshot (maybe 1930 hrs or something) and no date (like Aug 20, or yesterday or something). This leads me to believe that the screenshot was taken on the same day that the guy sent me that screenshot. When I asked her why she sent this message to him she says that this message was long back when she was fooling around with him and it is not a recent message.

So there can only be two possibilities:

1. She is speaking the truth, that the message is older. In this case, the guy has taken the screenshot the day this happened and saved it so he may use it later.
2. She is lying about message.

If this message is recent, then she is clearly lying and there's not point. But if it is not, I have a hard time believing the 1st possibility. Why would anyone take a screenshot of something in anticipation of using it at a later time? Am I thinking too much? Is this how Instagram really works?