doubting-a killer

I feel so bad when doubting. It's killing me because I know I am hurting my fiance who is trying his best to help me. He loves me so much and he's afraid to lose me. He told me that he's willing to help me and he just can't leave me. I feel so comfortable with him cause he lets me talk about all what I feel. But despite his great love I sometimes doubt him (you know I told you the story about me and my sister) don't get me wrong I just doubt that he's interested in her in a way (not more than that if you know what I mean) sometimes I laugh at myself because everything he does shows how much he loves me. He's tolerant with me but will he be always?
Please how can I stop doubting???? Is my doubting due to my childhood where no attention was payed to me?? what can I do about it?? sometimes I feel so selfish I ask for so much attention maybe because I didn't get enough when I was a child. Just tell me what can I do to help me