She left me alone and went with another person...

We where in love for 2years, Now she left me and she is in love with another person .... she is in my class only .... when ever i see her i get very much angry... i could not tolerate this... i could not do anything ... i am not able to think of anything except her betrayal to me.... my head pains like anything when i think of her and how she was with me.... i could not digest it .... i dont have any one to share my things... i feel so very lonely.... for past 2 years she was my world .... suddenly my world has become empty... but she behaves as if nothing has happened she is happy with him as she was with me now...!!
i am in depressed state .