Relationships and cheating don't exist together. A relationship don't allow the presence of cheating. Cheating causes easily the end of a relationship. It is considered as one of the most destructive behaviors to occur to a couple in a relationship.
You know, when you think about it relationships are the most amazing complexities. They are made linking two separate people with different values and perspectives on how things look and how they should work. Two people with very divergent needs and wants. Relationships are expected to flourish in the midst of blending all the distinctive and opposing conditions of each person. It is a wonder how any relationships survive. Many relationships do survive though and they can grow to be healthy and beneficial for both partners.
When a couple has come so far as to create a meaningful connection with each other then cheating is devastating. I am sure you know this. Sometime in your life you have either known someone who has been cheated on or have been cheated on yourself. Or...there is the other possibility...that you were the one doing the cheating. Whether you were the cheater or the cheatee (is that even a word?), it hurts.
The cheatee hurts because of the intense feeling of betrayal.
The cheater hurts because of the lying, secrecy and guilt.
It is a no win situation for all involved.
How will you ever get over it? Will it happen again?
The big factor here is going to be the trust factor. If you are ever going to be happy with your significant other again then either you have to forgive them and trust them or vice versa. This is not going to be an easy accomplishment. In some cases it may not even be possible, but that's something that the person who was cheated on has to consider and decide for him or herself.
The cheating situation could have occurred for a number of reasons. Possibly the cheater will say the affair meant nothing. It was a one night thing and it just happened. Just "happened"? I don't understand that myself, but if the cheater is rather immature then I suppose it's possible.
If you were having problems and the cheater thought the relationship between you was over then it could have happened. This is not giving them an excuse, but it could be the reason for infidelity in a relationship.
Either way the cheater was WRONG and they hurt the other person whom they are supposed to care for immensely.
The two of you must talk about it. Get it out in the open. Realize the cause and see if you can mend the relationship. In many cases, the cheating is not the real problem. Cheating is merely the symptom of another problem. Be honest with each other. Discover if there is something deeper, more problematic than just cheating.
Then the person who was cheated on has to decide whether they can forgive and trust again. It's not easy, but that is not to say that it can't be done. Many couples have managed to survive an affair.
Staying in the relationship when there is no trust is a miserable basis for a relationship though. The constant suspicions are agonizing and uncomfortable for both people involved. Understand the situation, look at it from the other's point of view and determine if there is truly a desire for reconciliation in the relationship and cheating situation.