I understand the unsupportive bf deal... ex. - quitting smoking is a tremendous burden (almost as much as smoking itself) because so many around me smoke. But it has to be done for you and you alone. Is it that your bf likes you a little meatier than you'd like? You didn't state your height, so I can only guess that 120 lbs is on the small side. 126 is truly not that much higher. When I was 120 lbs I tried desperately to GAIN weight -- to me, skinny ='d unattractive. You have to decide what's the ideal FEEL for you, and maybe not solely the ideal weight.
Dumping your bf is something only you can decide based on your personal life. From the brief issues you wrote, I can't fathom advising that lol. But do take the time for introspection to find your ideal YOU =)