Where does doubt come from? In the general sense of mind and thought. It's a feeling that nothing works out for a person and their life basically sucks the shit bag; no matter what they attempt to do. Often times erroneous comparisons are made with people they see in their life or online that achieved similar goals, but they feel they simply cannot measure up. They usually firmly believe and take to heart all the negativity from people around them. If you choose to pursue a goal and you end up facing obstacles self-doubt can manifest in your mind steering you away from achieving the intended goal. One must have incredible discipline and strong will to overcome obstacles. Live through the shit while you are on the way to the light. Reach greatness by not allowing the doubts to control your mind, perseverance is key to success.
Doubt essentially can be a real dream crusher and goal killer. Now, one might attribute life situations as a deterrent to accomplishing goals. Also, the thoughts develop in the mind, like “ oh, that's impossible I can't do it”. Keep in mind the achievement of making plausible dreams that more or less fit with one's cards of life a reality. Self doubt can be perceived as an excuse to remain in the rut or routine created by the individual, halting their progress in life.
Also, doubt should not be confused with failure the two are very dissimilar. Failure is generally trying to do something that may be new or familiar but, never achieving the desired or correct result. Doubt is thinking one is incapable of pursuing the task, due to a physical or mental challenge.
One must understand few basic facts: doubt will only become real if one projects and truly believes in it. Ultimately, it will lead to one's own demise or failure.
Dreams are just that unless the individual decides to make them reality. Remember, self doubt is nothing more than a belief that forms in the mind. It can only turn real if one focuses their energy.
Focus on a plausible dream make it a goal and pursue to the end without letting doubt take control and you lose out.