I've had the same or similar problem most of my life so can say a few things.
First - if you lack confidence in yourself it will help you to find out what is causing this. It could be psycological but with me it was more a lack of feeling OK in general. I wasn't really sick but had an underlying physical problem which left me feeling uncertain, as if I couldn't really depend on myself to do the appropriate thing at the right time. So the first step would be to check out your health. I did not know this until I was diagnosed with the problem many years later and learned how to cope with the physical problem. I'm still introverted but that's OK - that's just my personality type. I'm no longer as shy, that's for sure.
One of my friends trained to be a Process
Coach. This procedure helps many people overcome similar problems. If you can't find Process Coaching online let me know and I'll contact my friend.