One of the most difficult things for me to learn how to do was to control my thoughts. Even now after years of practice, when I'm really tired I'll start slipping back into a negative pattern of thinking and have to correct myself.
Controlling your every thought is difficul but with practice eventually you'll get to where it's done out of habit because you have changed what was a negative thinking pattern into a positive thinking patten.
For us to can change we have to change our thinking. If we're used to being negative about our abilities we want get anywhere. But if we notice that thought and consciously replace it with a positive thought eventually we change that negative behavior towards ourself and it will show in our behavior.
I practiced replacing negative thoughts with verses from God's Word, the Holy Bible, because what better way to train my thinking than with what God says and thinks about me. After all, when we are too weak to do what is right God covers us with his grace strengthening us, so that we can do the right thing.
So the next time you put yourself down remember that you can do all things through Christ who strengthens you, you are the head and not the tail, you are God's beloved child, you are a joint heir with Christ, you are covered by God's grace. Overcome your negative thinking patterns by bringing into subjection one thought at a time.
If you're mind runs away with you and you realize you have a horror movie or drama playing in your head, notice it. Then replace it with what God says is possible. For example if you are imagining that you get in a car wreck, notice that scenario going through your mind then imagine angels saving your life. If you imagine running out of money and not having enough food to eat, imagine God using strangers to bless you with the food that you need. If you are imagining yourself sick, imagine yourself miraculously being healed. Faith is believing and in order to build that faith you need to start thinking about what God can do for you and in your life and through you and remind yourself that you are God's child and he loves you unconditionally.
Take control of your life and circumstances. Change your thinking. Start thinking about what God said. Start actively believing in your thoughts and your faith will follow. Stop preventing God from doing wonderful works in you and your life. Start believing and professing truths from his Holy Bible over you and your life.