Signs of depression? Focus on my work.

My relationship with girlfriend is going day by day weaker. Most of the day i am thinking about her especially when i am alone at home in night. I could not able to stay without her thinking and this causes me to inefficient in doing anywork. I want myself strong as her by being not in a state of thinking all the time about her. This thing is happening since two years when she cheated me and we were went for a brakeup. But lateron i could not able to live without her and i gave her one more chance to prove her that she will remain loyal with me. Since then the life is becoming more worst because i am feeling very insecure by thinking that she might leave me again. This thing coming to my mind when i expecting anything from her and she doesnot filfilled that. I hope you will help me out from this problem asap as i want to focus on my work.