That voice is an extremely destructive force in our lives. It keeps us from being the happy and confident people that we should be. It constantly criticize us and compare us to "better than us" making us feel miserable, helpless and discouraged when something doesn't go as planned. There is absolutely no way to satisfy it as it'll always try to find flaws and make us feel bad about them even if we are doing well.
Twelve years ago, I went to a transactional analysis course for a weekend. It has nothing to do with finance! T.A. is a method created by Eric Berne in the 1950's to help people dealing with themselves and other people around them. It has been greatly helpful in my career when I had to deal with stressful situations and made me a more confident person and player. The main goal of T.A. is to help us shut down that non-stop critic in our head so we can fulfill our potential and enjoy what we do.
This constant self-criticizing comes from the education and conditioning we receive from our all well intentioned parents, teachers, educators, coaches, the society etc. Basically, to accomplish the impossible mission it sets for you, you'll have to these things:
1) Be perfect. "Don't you ever miss a note or have a tiny bit of fuzz in your tone!"
2) Be strong." Why the hell are you getting tired?! you're so weak!"
3) Hurry up. "Man! you're so slow! Everyone got it but you! What's the matter with you?!"
4) Work hard. "It's not good enough because the piece is too easy"
5) Please the others. "you're not allowed to say no!"
There are probably more but these are the most common.
Lovely program isn't it? The good news is that you don't have to cope with that. If you can recognize where it comes from, you can tell it: "shut up and leave me alone!" "let me have a good time and be myself!" "Go away! You're not helping at all!"
It is not always easy to shut this voice up in a society driven by performance, social status, competitiveness and money. This society wants results and it wants them NOW! It doesn't care if you're happy or not as long as you perform! It will make sure you feel ashamed every time you don't conform to its standards of performance, beauty, material welt, success, etc...
Being aware of this will help you break free from this tyranny and enable you to be yourself. Not a robot, not an alien, not a super-hero, not a super model, not the world champion. JUST YOURSELF IS FINE!
You will then become more confident as you won't let that bitchy critic undermine you all the time. You'll also have more patience as you won't expect results right now and allow yourself proper time to learn things without a bully whipping you to go faster.
This is only to give you an idea about transactional analysis. It would take a whole book to go over everything. You can find many articles about it online. Take time to do a little research and see how it can help you as a person and as a musician. It has changed the lives of millions of people including mine.