There are many ways to improve self confidence. One particular simple way is to look at the things you like about yourself. Maybe make a list of 20 or 30 attributes about you that you think define who you are in a positive way. Take one or two of those and see if there's anything you can do to reinforce them, then work on another one or two until the list is as healthy as you would like it to be.
Don't accept your own negative self-talk, like "that was a stupid thing to do," or "what a dummy I am," etc. Give yourself a pat on the back for some part of a mistake as being the best you could do, or the best you knew under the circumstances. Every mistake, or goof has something positive that you can pull out of it if you try. And everything we do or say can be turned into a positive lesson so you don't replicate the same things over and over.