Did you explain this to your boyfriend? Like a lot of things in life, there are people who have nothing wrong with them per se and others who have serious problems. I would let your boyfriend know about this. I have bouts of depression and go through periods of not eating because of stress and become uhealthily thin. You really have to spell things out to some people, because they might perceive the thinness, for instance, as a good thing or something minor.
I would also see if there's some kind of support group for this phobia, even if it's just internet, and really discuss all your fears about it with your therapist.