Wow! I definitely have to give my point of view with this question ..
Infidelity is undoubtedly a choice, it will never be a mistake.
When you decided to be unfaithful to your partner no one forced you, no one pointed a gun at your head, nobody said do it or someone will die, no, on the contrary you just let yourself be carried away for pleasure or you just let yourself be carried away, when this it happens you can come to feel guilt, but never ever, it will have been a mistake, but a decision totally.
There are people who blame external factors for having been unfaithful, that is, they create excuses such as "I was drunk" or "I was drugged" or "I was blinded by anger" are valid excuses for forgiveness of an infidelity, and believe me that is not valid.
Of course you have to think about the clear exceptions, as if you were violated, or if they really pointed you with a weapon, but those are very rare cases that are not achieved daily and those cases should be treated with professionals.
There is something that is very important that you know when you are unfaithful to your partner, trust is very easy to break and very difficult to win again, losing confidence can take your relationship to an abyss without exit. Forgive your partner may be simple but harder is to regain your confidence in it without any doubt, many people manage to "forgive" and be back with your partner but end up becoming jealous maniac, in a way that even you can not stand nor will your partner do it so if he did not let you be unfaithful he probably does it by having to endure obsessive jealousy.
There are couples who forgive and say "we're still together but I can not get that out of my head, I can not turn the page" and that's the worst thing in the world, since you live every day remembering that dark moment in your life, and every time that you see your partner's face you remember, then if you're always going to have a hard time it's better to leave your partner.
Of course not all stories end badly, an infidelity can occur for many reasons and does not always mean that the relationship is doomed to failure. Usually these errors fall after living situations of poor communication, problems of intimacy, discussions ... They are a wake-up call, the signal that something is not right in our relationship, but do not have to end it, it simply serves to rekindle the flame of the relationship and make everything better.
Those are all the things that could happen to you when you are unfaithful and that is what you should analyze at the moment of doing it, but if you do so, never blame the other person for that, or blame an external factor, or excuse yourself saying that the relationship I was not going in a good way, again remember infidelity is totally a free choice in which many people can be harmed or benefited as the case may be.
You should always remember that trust is a privilege and not anyone is sure to say "my partner trusts me" hey kid if you are really trying to be unfaithful analyze all the circumstances and who could hurt ... if you really see that your Relationship has no future. It is better that you finish it before having something with another person so you would avoid hurting people and you could avoid feeling guilty since after all infidelity comes the fault ..
That's the advice I can give you and I hope it really helps you, remember it's better to feel good and not hurt someone to hurt and feel guilty ... no one forces you to be unfaithful, you decide!
Regards!