I would try the "fake it until you make it" mode. When you look in a mirror, you think negative thoughts; change the behavior, deliberately seek out and look in mirrors (certain times of day) and smile at yourself and greet yourself kindly, as if you were looking at a friend.

You treat yourself badly but are wishing you felt a certain way, saw what others see, etc. The only thing you can control (we cannot control our thoughts or feelings!) is your behavior. Just decide you are a friend (do you care if your friends are "ugly"? No, they're your friends and you let them be who they are; give yourself the same right) and treat yourself accordingly.

If that seems too hard, start with thinking of yourself as a younger sibling your parents make you cart around with you and look after? You don't like it, resent that you are stuck with her but she's your little sister so you do as you are asked; the family connection and whole remembering-what-it-was-like to be her age thing, making it bearable and even, secretly, agreeable because you have it in your power to help make her life better/happier.