When you make up in your mind that something may not work, you have relegated it to failure. Just by merely thinking it may not work, the amount of effort that you will put forth is limited. Depending upon how likely you feel it's success will be will subconsciously determine the amount and the quality of your efforts.
Most things that are legitimate and worthwile require work... Usually lots of work. If you want to be successful in your efforts you must wholeheartedly believe in the future success of your efforts, otherwise you are doomed to failure.
If you really want to be extremely successful, do not select any options that you cannot put your heart and 100% of your efforts into.
"Thinkers Wanted!™"