Okay, I am going to give an alternative opinion to the majority I expect.
A one off, isolated, episode of violence is NOT okay, but it is also not necessarily the start of more, nor completely unexpected in the circumstances. People lash out when in severe mental pain. Finding out your partner slept with someone else is about as severe as it can get, and I doubt there are many of us here who, on hearing that, would not be tempted to react without thinking, and lash out in temper.
OP, your fiance sounds utterly remorseful about hitting you, so I suspect that it is not something he has ever done before, or is likely to do again, but the trust in you has gone, and that is something that is very very difficult to rebuild.
You have to talk to each other. He has to talk to you, even though it is going to be painful for him to hear and which is why he is shutting it away at the moment.
What made you sleep with someone else? Is it someone who is likely to be a person that you bump into all the time.. in which case that will make it all the more difficult for you to repair your relationship.
People are not perfect, sometimes we make huuuuuge mistakes. It can be salvaged, but it takes a long time, and neither of you can pretend it never happened, or turn back the clock.
What you can do is talk, and if you decide to try again, you have to promise that a line is drawn under this, and it is not something that gets thrown back in every future argument.