Getting used to blocking these negative thoughts is hard, but it's never impossible. Something that helped me greatly was taking buzzfeee quizzes (weird, i know), but seeing that I had positive qualities to describe me when I was feeling down made me feel happy and like i belonged.
Though, as a routine thing, have a hobbie. When u have freetime it's easy to get lost in your head, so read, paint, go boxing. Whatever you like to fill the time, bonus points if it has to do with excersize.
Try to surround yourself with positive people and cut off the megstive ones, and if that's not possible always remind yourself to never take it to heart. Listen to positive music, because music influences our world perspective, so in this case music that promotes self love and body positivity. Even if sad songs can be comforting, it's never good if you listen to them too much.
Lastly, compliment yourself. Everytime you feel like you look ugly or that nothing matches with your features, say the opposite of what you feel. For example, turn "I look fat in this," into "I'm beautiful but I could do better in terms of clothes". This will slowly help change your mindset.
I hope this helps.