A Change in Personality: Why is this happening?

Sometimes alone, its hard not to try to forget about Gu Gu. I always advise others who keep thinking of their loved ones to keep their mind focused on other things such as hobbies or their jobs so as to temporarily forget about their loved ones but then its actually harder said than done. Today at work, I kept thinking about Gu Gu, wondering how she is, whether she is feeling alright, etc. I took out my phone and looked at her picture and the memories flooded me. The first time I held her hand, the first time I said 'I love you' to her, the first time I kissed her, the first time she leaned against me. I had mixed feelings, happy and sad. Happy at remembering the times when we had so much fun, but yet sad that somehow, I'm beginning to feel distant from her because of my work. I hate leaving her alone, in the company of other people. I hate the feeling of knowing that she needs me by her side, yet I'm not there for her. I just feel that I'm not giving her enough happiness, not giving her that feeling of being loved. Hais.
Secretly, everytime she goes out with other guys, I get this feeling of sadness. Its not that I don't trust her, its just that I feel that she is spending more time with other guys than me, and somehow, I'm getting jealous. I always told myself that I should give her a sense of freedom, to make her own friends, have fun, but somehow, everytime she goes out with other guys, I get a sense of resentment building up inside me. I believe that trust is essential in a relationship, but somehow, I don't know why, I'm starting to lose trust. Not in her, but in the people that she goes out with. Anyone, even the people I know, I start to build up a protective shield against them. A********, B******, C******, I used to be friendly with these guys, especially C******, but somehow, I'm starting to be wary of them.
I feel that I'm starting to lose my friendliness, my attitude towards others has started to change. I'm losing my nice-ness, my good-ness, I'm changing. Why is this happening? I start to feel that everyone is becoming my enemy, and that I can trust no one. I'm starting to get negative thoughts about everyone, no matter what good friends we were in the past. I start to have this feeling that everyone has a motive to do me in, to destroy me, to see me lose everything I have and day by day, I start building up defences against them. I'm starting to become sick....no that's the wrong word...I'm starting to become evil....