I have to stop worrying so much about my physical appearance and develop a backbone.
この場合、次のような言い方ができますよ。
ーI have to stop worrying so much about my physical appearance and develop a backbone.
「外見を気にし過ぎるのをやめて芯の強い人にならないと」
physical appearance で「外見」
to develop a backbone で「芯の強い人間になる」
ご参考まで!