From where I sit, if you care about whether or not you are the "worst girlfriend ever," then you most likely aren't. People who are truly awful don't care about their effect on others.

I relate to the feeling. I am disabled both physically and psychiatrically, and therefore I am unable to hold a paying job. My husband supports me, and he works so hard doing it. I often feel that I don't contribute anything worthwhile, and he'd be better off without me, but oh boy, is he quick to disagree with that if I were to say it. I'm sure your boyfriend finds more benefit to having you around than to being without you.