why do i feel down in my new relationship

I have been with girlfriend for around five months and we love each other very much!Thing is I have been feeling anxious recently about the relationship, and i cant really figure out whether its me or the relationship. Everything is alright when we are together, but when im not with her I feel like im waiting for her to phone me.
I start to worry about her with her previous boyfriend, and that all the stuff she is saying/feeling/doing with me is what she has done with someone else before, which makes it feel less special!
I am also very dependant on my girlfriend at the moment cos not a lot of my friends are around at the moment, which puts more pressure on the relationship to work!
I just want to stop worrying and enjoy the relationship, how do i do it? Tweet