How to stop being so insecure?

Hey, my name is Katie, I'm 20 years old, bisexual, and I have a brilliant girlfriend, who I absolutely adore. I am very insecure though, due to the fact that I am permanently in a wheelchair due to spinal-cord damage. I am in perfect health except i can't walk. My girlfriend is great with me being disabled; she sits on my knee when we kiss, pushes me around when I get tired and happily cuddles me in bed because she knows I can't have an orgasm. She is brilliant, but I can't ever shake the feeling she is going to get bored or frustrated with me and leave me for some other woman who can walk and have orgasms; I constantly feel like I don't deserve her and get really scared or miserable if she goes out with other girls to places where I can't. I don't want to feel like this, its selfish and pitiful, but how do I deal with my insecurity?