Fake it until you make it! That's it. Believe it or not, everything you say stays in your subconciuous. If you're constantly saying yourself you can do it, you're worth it, you are great, you'll start actually believing it.
Everyone has insecurities but some are more aware of them than others. I always had really big ears and it had never been a problem until elementary. There was a guy who bullied me about it and always said no one would like me becuase of that. As dumb as it sounds it made me really shy. I had a hard time making new friends and I was relly aware of how I looked and what others thought about me.
It wasn't until middle school that I realized that people saw me the way I felt. Then I started faking I was really confident, even if I wasn't. I think posture is a key. If you walk straight and looking up, not to the floor you seem that you know here you're going. My mom says, you look like a princess.
Other thing that I think really helped me was practicing. Whenever I went to the market, coffee shop, or anywhere else I would do small talk with the cashier or just some random person there. This really helped me with my social skills, I wasn't afraid of saying what I wanted or speaking in public and making new friends.
What do you think works too?