I do not understand......

Hearing the news that she has broken up with him STILL came to me as a shock even though i already knew what she was going to do 3 weeks ago. Nontheless, i was speechless. Maybe it's because i was thinking about him at that time. I wondered, how he is and how he would've have taken the news. The instant i heard that he was crying, a small tear shed from my eyes. I wanted so much to call him and comfort him and let him know that i am there for him. But, i was to pretend that i knew nothing about it till he tells me.
Not long after that, he came online. I saw his broken heart and asked casually if he was alright. He told me the whole thing. That she left him for another guy. There i was thinking, she wouldn't do such a thing. She told me she knew how it felt like and she would never do such things to him.
Later, i spoke to her and yes. She finally told me the truth. She had another guy. In an instant, i thought, what a bitch. Lying once before and now...... I was speechless for a second or two. I was not angry at all for what she has done. I doubt i knew what was going on in my head let alone my heart. But they both tell me that i shouldn't be angry at her for her actions and not to hate her. I did not know why.

I have felt the immense pain of having someone take someone away from me. And she was the one who did it. And now here she is, doing it again to another girl who was in a 8 years relationship. I was thinking to myself.... What is she trying to do? Doesn't she understand the pain?

The only thing my heart is telling me to do is i should congratulate her with all my heart and be happy for her. But at the same time, warn her that if that guy can leave a 8 years relationship, anything can happen between them now.

The thing that i do not understand is, how come i am not mad at her? And why am i being so calm about all this? The fact that i kept telling her that i understand and i do not hate her confuses me more.

Do i really understand how she feels and at the same time understand the pain that she has caused the girl and him? Am i turning into a "i've experienced it and i understand" kinda person to the extend of forgiving everything that has gone wrong? Or am i going mad?