Women don't want to be the bad guy. That's why they always want to be friends, always want to make sure you are okay. They even convince themselves that it's better for you if they lie about the guys they were shagging behind your back.
All that sex and fun the week before? She was all happy and excited by her new emotional relationship, and you were experiencing the reflection of that. Then she decided to go for the real thing, and it forced her out the door (behind your back) into the new relationship she wants to pursue. You're now just her backup plan, and now she doesn't even have to feel guilty about her actions.
Of course, that may not be true. She may be honest and truthful with you, but that doesn't sound like a person that sneaks out of an eight year relationship behind your back with a cursory note, and new male companion in tow. You don't do this on a whim, you think about it and plan it in advance, and hide all that deviousness from your current partner. Just like the sort of person who comes back and lies to your face about it all, because it makes their life easier.
Even if she didn't sleep with the new guy when she says she didn't, she is now. You're officially broken up, because she made it that way, and now she can pursue the new guy. A bit of new attention has turned her head, and she's blown up her relationship with you because she wants the freedom to have the new guy.
Follow the previous advice. Throw yourself into work, hobbies, gym, travel, new people, socialising, etc. Look forwards, because you can't go back. She doesn't want you any more, she wants someone else.