My boyfriend doesn't want sex anymore. Is he not in love with me anymore?

It might be painful but I think you need to get to the bottom of the 'why do you think that is' comment. Something is in your boyfriend's mind and, as it won't just go away, I'd suggest you bring it into the open and both try to deal with whatever it is. But I would warn you, it is not likely to be easy or painless. Good luck.