I have news for you: MOST people try to appear more confident than they really are. In my line of work (I'm a counselor and certified professional Life Coach) people tell me things they would never admit to anyone else. The bottom line is that insecurity, fear, anxiety over a million different things are being carried around by a great, great many people. The difference between those people and you is that you are willing to admit your lack of confidence.
So, none of that really solves your issue does it? Of course not. I would humbly suggest that you focus on those parts of you that are strong, positive and self-assured (no matter how tiny that part of you may be.) Like Oprah Winfrey famously said, 'whatever you focus on grows.' So why not spend more and more time each day thinking about all those things that make you feel special in a good way? Convincing yourself there's a reason to be confident takes time. There's no magic bullet.
One last thing. Everybody makes mistakes and fails. EVERYBODY. If you're not missing the mark on a daily basis, you're definitely not a human being. Accept every mistake you make as a course correction. A training exercise.
Wish you the best.