Lied and need help

I've been with my boyfriend for almost a year and when we first met I lied to him about my age. I was 17 and told him I was 18 just out of stupidity. Now I'm 18 and he thinks I'm 19. I know it was stupid to do and i have no logical reason why I did it. How do I tell him? I think he's going to be really upset. :(