Don't give up! Please don't give up. Let your boyfriend say whatever he likes and don't let it get to you. If he really has a negative aura tell him you need some time alone. Don't worry, he if likes you, he won't break up. You can never lose something that's meant to be yours. Chances are, he will respect you all the more when he sees how serious you are about this fast. And don't forget that fasting is also about cleansing and meditating. He might not understand that, a lot of people don't practice this.
All my Love and Blessings.