Forgiving is always easy. How long it takes to trust again remains for another topic. You may be the cause of why it happened, but never allow that to be the reason why cheating is ok because it isn't. If you weren't good enough, the cheater should call it quits instead of justifying reasons why he/she cheated. He/she took the easy way out to seek physical and/or emotional relief.

You may reflect on yourself, for improvements, but don't take the blame for it.