You are insecure. Don't worry a lot of people are insecure. I used to be the same. It was like whenever I was inbetween relationships I felt really down about it.
Insecurity is a feeling of general unease or nervousness that may be triggered by perceiving oneself to be unloved, inadequate or worthless.
http://en.wikipedia.org/wiki/Emotional_insecurity
You have to learn to be happy with yourself. Focus on your interests and passions in life. I realised that there are so many things that I am interested in and every girlfriend I have had has held me back from doing these things. I don't even care about having a girlfriend now. If I did have one, I would make sure she didn't hold me back from dong the things that I'm passionate about.
Do some reading on inner game and learn how to overcome your insecurities. This will greatly improve your game because if you are gaming at the moment your insecurities are probably given away to the world just by your body language. If you have great inner game you will have great body language and girls will come to you.