It might have been long that you are trying. You might not have got the success you wanted till now. The thought of quitting must be hitting you again and again. A voice must be telling sometimes "its enough, probably you do not deserve". Do not listen to it. Try to listen to the voice which have been telling you all along " YOU CAN DO IT". This is your real voice.
You have not got the success, that's true but you do not know how far you are from the success. There is no measurement of effort. The only unit of effort is "keeping moving". Do not stop until you succeed or there is a compelling reason for you to stop. If you really put your heart and soul to something then its impossible that you will not get anything. It may happen that you did not get exactly what you wanted but you will definitely get something on similar lines.
Your effort will never go waste. You must keep moving ahead for what you are looking for. Everything may look dark around but you must believe in yourself. Whenever the thought of quitting comes to your mind, remind yourself the reasons you chose the path and how will you feel once you achieve what you are working for.
Do not quit! Do not quit! Do not quit! You will reach your goal!