There could be any number of secondary factors but the primary factor that determines your success and failure is you.What your subconscious mind really wants decides the outcome and it overrides every hard slog and smart moves of yours.
One could consciously wish, pray and work for success but if you subconsciously carry a lingering doubt it would eventually play havoc. You could fall sick, run into unexpected situations, commit the unthinkable mistake or just about anything to make sure it is not favorable to you. The rules of destiny are simple. You always get what you want. Fortunately or unfortunately what you want subconsciously, that you had built over many many years from past actions and results, far outweighs what you want consciously ending in results that you may or not cherish.
The subconscious scar could be because of fear of failure, fear of success or strong desire for self deprecation and self pity or simple wishful thinking. Whatever the reason be it is enough to act as brakes to stop a success run. Though positive thinking may appear to alleviate self doubt in the short run, it only ends up colorfully clothing it. Self talks and loud expressions is not going to fool oneself for long. It would slowly begin to wither and drain you.
The simple solution is to take your legs of the brake instead of hard pressing the accelerator in the name of positive thinking. When you realize that you have created the brakes it would be lot easier to drop them. The momentum you would gain then would exceed your expectations and surpass many milestones.