Most Helpful Guy
There is a lot of ambiguity in what you have stated here, but i'm just going to go off what I have seen from your question.
Firstly, it is needless to say okay to be hurt and angry. But you need to clarify one thing, did he actually 'cheat' as in kiss\inappropriately touch\have sex with another person? Or was it highly inappropriate behaviour by overtly flirting with other women and\or proporting himself to be single?
I know that there is a disparity here in terms of how a man vs. how a woman sees these two things, however I have found, learned and studied that whilst grossly inappropriate, flirting with other women is not as is traditionally labelled, 'cheating.'
It is a sign... a sign that there is something that the man is not happy\satisfied with, in his current partner and relationship. Please note I tie the two together partner AND relationship.
It is also a sign that the man is not doing the right thing by trying to discuss it with his partner. Although it may be an uncomfortable conversation... fact is, if he respects his relationship and his partner he should at least try to discuss it. But in fairness, sometimes his idea of trying to talk can vary very much from how a woman would consider talking.
It can be a very destructive and damaging behaviour which erodes trust and intimacy within the relationship and for the other person in the relationship. But it can be overcome with a lot of work, professional help and commitment.
Moving on - actual cheating can be attributed to the same thing (not always) just in a more significantly damaging way.
If this is the case, well - I am not so sure as to why you want to demonstrate style and grace... what I would recommend is just be professional, especially if he doesn't know you have found out.
Stating something like, "John, I know you have had sex with someone else. That is a deal breaker for me, the relationship is finished. I am quite hurt by your actions, however I am committed to this decision - please don't make this situation any more difficult than it has to be... I wish you happiness in your future." and leave it at that.
I am aware of how this would read and even sound... but acting cool, collected and absent of emotion will signify you mean business whilst also showing that you aren't going to succumb to emotions; which could in a way, achieve that style and grace youve been searching for.
Hope this helps
All the best in your future.
Most Helpful Girl
Go on and live the best life you can for yourself. Forget about that ridiculous boy.