Jesus warned his disciples, his friends, that they were going to deny him, betray him and fall away. They all denied his predictions. In their hearts they sincerely believed that they would never do such a thing.
It wasn't until it was over that they realized what they'd done. It wasn't until Jesus was being dragged away and looked into the eyes of Peter that Peter knew that Jesus was right. He did deny him. It wasn't until Judas witnessed the violence of the guards arresting Jesus that he realized what he'd done. He did betray him.
Most people would never betray anyone. Like the disciples, we take one sincere step at a time, tiny little concessions that aren't much until the final act is complete and the betrayal is betrayed. Peter only wanted to be as close to Jesus as he could during his trial. I suggest Judas only wanted to propel Jesus' revolution into the core of the power-broker's headquarters. But suddenly their denial and betrayal was exposed for what it was. Even though he didn't know it, Peter despised the cross. Even though he didn't know it, Judas didn't understand Jesus' revolution. The revelation was a terrible one. Peter fled and wept bitterly. The others ran. Judas in inconsolable anguish hung himself.
Opposition from enemies is one thing. That isn't betrayal. Betrayal comes from well-meaning friends and loved ones. Almost all affairs start with innocent friendships. But somewhere along the way a vital line is crossed where it morphs into betrayal, often without the conscious will of the offender.
This is why betrayal is so easy to give: we often sincerely don't realize it until it's too late. Betrayal is hard to take because your friend, your loved one, did this to you. And this is why forgiving yourself for doing it is humiliating: you didn't know you were capable of doing something so cruel. This is why forgiving someone for doing it to you is necessary: most of the time they didn't mean for it to come to this. They didn't know what they were doing.