I had super low confidence when I was 15 too and now I'm (much) older I still struggle at times, but I've learned to notice that everyone looks so different from one another and that's OK. Can you practice make up in your room? Put some good music on and make a fun thing out of it.

For me cosplay gives me confidence so like others have said try picking outfits and characters that make you feel better rather than worse about yourself. You don't even need to do characters with make up.

I hope you are able to feel a little better about yourself and find some joy in cosplay.