In his book, The War of Art, Steven Pressfield managed to detail the biggest obstacle to you achieving your dreams in one word: resistance. Steven writes:

"Resistance is the most toxic force on the planet... To yield to Resistance deforms tour spirit. It stunts us and makes us less than we are and were born to be."

The reason you haven’t finished the big project, lost 25 pounds, or never finished the manuscript is because of resistance. It appears in camouflage, acting like support, rationalization, and common sense. Simply put, it keeps you from the work.  And the resistance biggest tool against you? Well, that's you. Resistance will tell you that you are not good enough.

Why else would you not do your best work? Self reflection counters resistance.  

Self reflection is the simple idea of auditing your habits, actions, and thoughts. It could be a habit of late night eating. It could be a repulse when someone critics your work.  It could be the idea of perfection (waiting for the right time to finish). Define the enemy. Realize you can overcome resistance. In other words, know you can live your dreams.

"There never was a moment, and never will be, when we are without the power to alter our destiny."
- Steven Pressfield, The War of Art