It's partly your fault that your chick couldn't keep her legs closed and be faithful??? I THINK NOT!
Seriously the fact that you think it is in any way your fault is the most concerning part of this story. That is utter BS. You were together 4.5 years, when the going gets tough................ she gets into someone elses bed. UNACCEPTABLE.

As for my general opinion of cheating: 'Adios and shut the door on the way out'.

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You never know............................. until you ask!