Stand tall when you walk, carry yourself with pride. Have conviction in everything you say. Don’t take rejections (of any kind) personally. Don’t change your mind about something just to fit in, have an opinion and stick with it (without being stubborn)
If this sounds difficult, just fake it. Pretend to be confident. Act like a more confident person and after a while you’ll realize that you’re not acting anymore.
Check out the book Developing Your Confidence by Cliff Boer. I read it and it helped my a lot.

Think positive always.