Why do i have such low self esteem?

I think im so ugly, and even though people say im not i feel like im so ugly inside and out. i hate it. im in a relationship its been 7 months and hes the most amazing guy ever. we've been together for 7 months and hes such a faithful caring honest guy. I know hed never cheat on me but i just get so scared that one day hes gonna go back to his ex. he tells me he doesnt like her but shes going around starting **** about me and i really dont like her, and sometimes i feel like he stands up for her instead of me and i get so insecure about myself and his feelings for me. why do i feel like this? i dont want to = (