What drives us towards achieving something? It is very intricate to define realization, satisfaction and happiness, but there is clearly a strong connection between these feelings. The goals we set to ourselves are for the purpose of attaining happiness, whether in the short or long run. Once a goal is achieved we feel satisfied which makes us feel a sense of realization, and this makes us happy.
In theory this all sounds crystal clear and even straightforward, then why do we find some goals hard enough to make us fail to achieve them? There can be many reasons one does not feel capable of reaching something, but there is only one real reason that can turn your world upside down, motivation.
Motivation is probably the hardest thing to achieve but yet the most powerful weapon anyone can give to himself or herself. Motivation is the key to achieving the goals you propose yourself, it is the engine that powers your brain and the fuel that drives your body. Motivation is not a one-day inspirational thought; you require real meditation and thinking. Motivation is a plant you water every day, is a mental state you develop every day.
Ask yourself, how bad do I want this? And your answer should always be: really bad. Motivation is the ability to sacrifice, at any moment, what you are for what you could become. It is the consciousness that no matter the obstacle, the effort or the time required to achieve your goal, you will make your way up the hill inch by inch until you get to the summit.
Most of us go to sleep not feeling really tired and wake up to the sound of our alarm clock wanting to sleep for, at least, another 3 weeks. Most of us have fantasized about how much we would like to swap those situations; going to bed dead tired and waking up ready to take over the world.
This does not mean you will not fail, you will and this is fine. As the athlete Abby Wambach used to say: Sometimes when you fail, it allows you the opportunity to grow more motivation and get more intense about your training.
For every goal you set yourself set your mind to it, think deep down: who you want to be and who you want to do it. Never fear failure, as it will only make you stronger.

Additional resources:

How to Overcome Failure
Learning from Failure
If You Fail to Plan then You Are Planning for Failure