Why do I have such low self esteem?

I'm 19 years old and I am biracial. My mom is native American Indian and my dad is black and Cuban. People are always complimenting me telling me how "beautiful" I am. I feel so ugly. Everyday that goes by. I avoid being infront of people because I feel so uncomfortable. My boyfriend tells me I'm beautiful all the time but my inconfidence is ruining our relationship. I don't know why I feel like this but I hate it. I wish I could feel just as confident as other girls. Why am I like this and what can I do to change it??