Our ability to reign in our thoughts and not let ourselves lose control over what can be accomplished takes practice.  We are our own worst enemy at times, when it comes to questioning ourselves and self doubt.  Since it has taken many years of practice, be prepared to spend some time to unlearn this behavior.  The good thing is that you can get rid of old habits and you can learn new positive one's to replace them.  Diligence in seeking to be constant and resolved about this is what enables us to do so. When you have those days that you question your ability to achieve, remember that it takes one small step daily to get yourself to the right destination. As long as you are pointed in the right direction and keep walking forward, you will arrive at your desired goal.  Don't allow yourself to question your capability and leave self doubt in your past where it belongs.