Delete her off Facebook. No CONTACT all the way! All the time! Learn to Hate her. Forget about her. Delete and throw away EVERY single thing that reminds you of her. If she is with the other guy that fast then there's only 1 reason for that.
She was EMOTIONALLY CHEATING on you before she actually did it. Her saying she's doing it to move on is BS!! She had that thought and maybe was even with him already way before you 2 actually broke up. That's probably why YOU BROKE UP in the first place.
The grass is not always greener on the other side... It only seems that way at first glance, but the fool will never realize this until its too late.
Let her be! Leave her alone. Learn to hate her! Better YOURSELF as a mAN! Work out! Buy new clothes. Learn new skills! Be a new man! Change yourself within for the better. BETTER YOURSELF and in the near future, when she sees you again, she will see what she missed. By then you will not want her anymore... I can't stress how important this is!!
By doing all these things, you will have done 2 things!
1) Become a stronger, smarter, mentally tougher man to prepare you for the next relationship.
2) Turn the tables in your favor because if the love was real, she'll come back begging for YOU! And by then, it'll be up to you to choose if you still want her back and IN YOUR TERMS!. But you must also remember that she might never come back... And in that case, HER LOSE...