Fear of success, or FOS, is one of the most popular, and misrepresented theories in psychology. Many people actually BS themselves and others by using FOS as a reason for not being diligent, dedicated, or focused - while arrogantly claiming that they could succeed, at any particular endeavor, if they chose to.
"If" is the operative word.
Fear of success is not fear of failure. As the name suggests, it implies that one is fearful of succeeding at something, or in some endeavor. Psychologists say that is characterized by a lack of belief in one's ability to sustain progress, or that one will not find happiness in one's accomplishments (if they were to attain them).
The main point - and this is crucial - is that real fear of success means that you are fearful of the consequences that you think will come with success; therefore you subconsciously avoid becoming successful to prevent those consequences from occurring...consequences like having to work even harder to maintain your success, or the loss of freedoms, options, etc.