Hi,
First of all, I want to say that I feel very sorry for what you are
going through. But I would like help you see things in a more
positive light.
First of all, since your gf "walk" with this new guy so soon after
breaking up with you, suffice to say, she probably have the
intention to be with him when breaking up with you.
Instead of being upset with her for "lying" to you. Why not see
things in better light that she didn't want to hurt you by saying
she is breaking up with you for that new guy.
Seeing your extreme "reaction", it is understandable, that she sort
of tell you a white lie so that you can have your pride.
As for her telling you that she is with the new guy, she is
probably saying that so that you will "give up on her". She already
had given up on you... so you should just close the book on her and
move on.
She broke up with you to be with the new guy, isn't this more noble
rather then trying to double time you and him? so in a way, she
seemed like a nice person.
You should deep inside your heart wish the two of them well. As for
you, you shouldn't take things too personally, a r/s needs mutual
affection and attraction, maybe she is just not the one for you.
Why not wait and seek out another lady that feel for you the way
you feel for her....
Try to be positive, and you sound young, there are more
opportunities down the road....