Sanji needs to love himself as much as he loves others(mainly women with the small exception of his crew and Zeff), that way he can access his full potential.
Sanji needs to accept himself whole, the good and the bad. He's ultimately Zeff's son, the good side he already in peace with, but also Judge/Germa's son, the bad side he rejects. The two come from a place of love, but while one one is accepting, the other is imposing.
When Sanji makes peace with himself, he'll release his shackled inner self, his CoC, and only then will he fully conquer his Germa Lineage, his Ice cold heart, and his Sora inheritance, his fiery heart. With both he'll have the strongest body and shield for defense(total fire immunity and invulnerability), and the strongest fire and spear for offense(Black/Pink flames of destruction).