What can I do to stop feeling like this ?

Lately I'm worried about how I've been acting around my boyfriend. He's such a great guy, charming, kind, romantic - but he's not ALWAYS there and for someone who's been let down in the past this can make me feel paranoid and anxious. He does work an awful lot and is devoted to one of his pastimes so I understand he needs that sort of "Me" space. Lately a girl i know has been flirting a bit with him, he jokingly plays back but I had a conversation with the girl about my insecurities as id already said to him and he said he felt nothing for her. She told me that he loved me and she was in fact jealous of what we had, she said we were so sweet together and i was so encouraged she said this, it changed my opinion on the girl. But then if someone leaves a comment on a social networking site (be it a good looking girl) or if i see him in pics even with a male friend i get so envious. I suffer terrible low self esteem and am getting councelling at the moment. I've been acting a bit young as well and sort of huffing sometimes to get his attention which is working, but i feel like a stupid little girl! how can i stop this? please help! im 18!