Hi Sorry,
I understand your feelings and can see the dilemma here but honestly I have to say that 18 is a bit young yet to be ready to send your family packing, over a boyfriend. I know you wouldn't be the one cutting ties but by choosing the boyfriend and knowing the consequence of that, you'd be choosing him, over them, in a sense.
It may be that some day if they don't soften toward you in regard to your not wanting to be a JW. you may have to make a difficult choice. Right now, insomuch as you're so young and haven't known this guy all that long, as well as the fact that you're quite torn as to what to do, I'd say you should give this more time. This isn't an emergency although at you're age, it might feel more like it is, than it would if you were 21 for instance.
A year ago you were in a place where you thought getting baptized was a good idea even though you had a hunch that things weren't all they seemed to be with the religion. You mentioned that your boyfriend isn't in the best place right now and given the fact that you've (wisely) come here for advice, I'm guessing you also may have a hunch that doing anything too drastic until you've known him a good long time, wouldn't be a good move.
Why not take a wait and see attitude....let this play out a little before you go all in with your boyfriend? I'm sure his Mother is quite fond of you but to think that she loves you dearly this quickly, might be a sign that you could putting a rosy romantic glow on this new relationship. People can love you dearly, then not love you dearly, just as quickly. I don't know the situation but the mother may well see you as the answer to her son's problems and a way to get him off her hands.Your parents for all their faults have been there for you from the beginning and it sounds like you love them very much. As JW's they have learned to be terrified of the idea of a child not remaining in their faith. It would almost be like a death to them to have you leave.
Perhaps if you get to know this guy within the framework of your existing situation and ease your parents into the realization that their little girl has a mind of her own, you can spare them much grief. It will take patience and maturity but things would be some much better if you could avoid a dramatic scene where voices are raised, doors are slammed and the sound of horses hooves over the frozen ground as someone gallops off into the night on horseback.