^^^^^^^^
There is your answer.
You need to FIND OUT WHY.
Or else it will forever haunt you, and she WILL cheat again.
Ask her why did she do it, based on her answer you will get your answer.
A woman does not just wake up one morning and decide to go blow and F an guy.
In addition it happened twice, in the space of two months.
Do not accept " I don't know " as an answer.
Also forget that crap she said about " he was a lousy lover."
If he was so lousy, then why did she go twice?
Are you sure she only went twice?
Demand the truth.
Time, Place , everything.
Get screened for STD's.