Du'a (prayer) for when looking in the mirror

الْحَمْدُ لِلَّهِ , اللَّهُمَّ كَمَا أَنْتَ حَسَّنْتَ خَلْقِي فَحَسِّنْ خُلُقِي

Alhamdulillah, Allahumma anta hasanta khalqi fahassin khuluqi

Meaning: All praises are due to Allah, O Allah as you have given me a good physical form, so also favour me with good morals and manners.

Try posting it on the mirror that way it becomes a habit. Everyone is beautiful in his or her own way, after all Allah swt has created you and he is the best of the best.