I have been exactly where you are bro. First excuse me for my English because I am not a native speaker.
so here is why you cant get her out of your mind, you are more focused on her life than on your own and you are jealous that she is happy and you are not. I am sure if you knew she is suffering for ANY reason, you would feel much better than you are now.

My advice and what has worked for me, focus on your own life, dress well pretend you are happy, keep yourself as busy as you can, join a gym, as they say when a heart is broken a body builder is born, eh? so build some muscle, groom yourself and go out there, girls would die to get your arms around them.