Self confidence help?

Some days I feel good... other days I just want to ball up an die. Im 17, 5'6-5'7ish, and last I checked I was in between 147-153 pounds.. I feel horrible about myself. My boyfriend tells me im beautiful an that he loves my body. But sometimes I feel like he's not attracted to me anymore. Is there any tips that actually work? I have such a low self esteem and I hate it cuz im constantly picking awful things about myself.