Believing and achieving are two different words entirely, Believing is like a 'want' while achieving is like a 'need'. It is possible for you to believe and not achieve.
I read a story of a guy who graduated with a distinction and had the hope that he was going to get a good job and move forward in his career but along the line, things did not work out as planned. Someone advised him to try getting a schorlarship, but he felt he was not up to the standard since he had tried and nothing was showing forth (believed) achieving will be a little difficult for this type of person.
Believing and achieving work together if you want to be successful, there is no dream that can not be acheived with hardwork and persistence. Create your scale of preference and work towards it. i know of someone who worked in a company and he felt the knowledge he had was low compared to his colleagues but he always say something i love, 'i know i will get there' that is believing and achieving and being successful will not be far from such a person . Believe is accepting while achieving is successfully bringing it about. Remove the negative voices and focus on your dream. Believe with all your heart and achieve with all your might.