I think the best way to gain confidence is by taking pictures of yourself. No matter what you’re wearing no matter where you are, snap a shot. You can see yourself in whatever relaxed position you are in and get an idea of how you’d look to other people. You can find out what expressions and poses flatter you the most and see that even when you think you look like shit, you’re actually still pretty hot.
Pro-Tip: Keep a special-secret folder of nudes on your computer for only yourself to see. Take pictures of every single piece of skin you’re insecure about. And look at them. Tell yourself again and again that those parts are beautiful. Even if you don’t believe it at first. Tell yourself that same lie…and one day, it won’t be a lie anymore. :)