Struggling with Self-Worth in a Great Relationship

Not sure what disorder this falls under, but I'm fairly sure it's some disorder because I don't always feel like this. most of the time, I feel the opposite. but tonight I'm drowning. my relationship is pretty amazing. my boyfriend has never done anything to have me doubt him. but, I've been with other partners who destroyed my self image. I just can't always feel like I'm worthy. I'm struggling so hard to lose weight, finally working out and working on my diet habits. but I can't shake the feeling that I'm just not worth it. other girls are smaller, more attractive, don't have mental illness. I just feel so replaceable. the annoying part is some days I do understand my worth. I understand that others could be more attractive but they'd never have my amazing personality and sense of humor or heart. him & are so compatible on so many levels, and yet I let physical issues take over. we don't even argue, we've had a single fight in the 3 and a half years we've been together. yet tonight I'm just fighting back tears with the irrational feeling that he's just going to leave me, just wake up one day and decide I'm not good enough.