Faking it until you make it really is all you can do if you didn't grow up confident. In the beginning you obviously won't feel confident like whatever you're saying, it'll feel weird and maybe even wrong, but repeating compliments to yourself and taking note of little things you like in your appearance (usually during mornings when you see yourself in the mirror and are about to start the day) can build up. It worked for me and some of my friends so I can say that it absolutely does have results.
Also, practice taking photos that utilize flattering angles. Insta models wouldn't be nearly as sexy/pretty if they didn't have their angles and filters, so just try and do that and DON'T get discouraged if you feel ugly in the first few tries. Doesn't happen overnight. Best of luck.