The following is a theory formulated by my friends and I (mostly UglyCasanova) during a discussion that started with girls and ended at life.
There are two internal factors that limit a person from achieving their goals. Laziness and fear. Everything that you want to do, circumstantially can do, and still don't do can be traced back to either of those two factors.
Imagine that you need a job. You are mentally and physically capable. You have useable skills. Yet you continue to sit on your ass and live in poverty. What is keeping you on the couch? a)You are lazy. It is much easier to stay put than to exert the effort required to reach your goal. Or b)you are afraid. The threat of failure is so great as to discourage you from trying.
Of course there are other reasons that things don't happen. There are some things that you can't control. The hardest-working, boldest person who came out of school in November of 1929 would have had a hell of a time succeeding in business.
Those people who are able to overcome their laziness and their fear are the people for whom there are very few limits.