I think that if you really want the diploma, if you can listen to the diploma level music and 'feel' it or hear it inside, if you can look at the music and begin to understand it, then you have the ability to achieve it. It might take you a while, with exploring a wide range more music to develop your playing (not just learning the diploma pieces). You need to work out what areas of your playing need development - is it control, expressiveness, speed, etc. Give yourself some time to develop and feel confident. Deepen your experience as a musician. IF you have the passsion, you need the know how. The grade is not a measure of your potential, but of your performance at the time - just as Rhapsody said.