You see yourself differently in a mirror lol. It's a reflection. What you see in pictures is what you actually look like. Soz.
You should listen to christina agulara's song beautiful. And then open your bedroom window and sing it loudly.
I think its just all about our mind? If we take a pic through camera (not with front camera) we usually firstly think about 'do i look good now? Is my hair messy now? Do my cheeks look fatty chubby at this angle?' →with these bad thoughts ofc we cant express our emotions well nor give out our best in front of camera which makes our appearances look bad in pic.
I know someone that is awesome at taking selfies it's like some hidden talent but when you see her in rl her selfies do her a favor. Not that she is ugly but it's better to look amazing in rl rather then photos, so if you think you look pretty in the mirror that's I personally cannot take good selfies the camera hates me and I personally think I look good in a mirror rather then photos.... Like all photos lol ?