Essentially, this is an advocation to the reader to challenge him or herself. Eliot is saying that we are always capable of more than we think we are. Only those who are willing to keep going to a point which they fear is "too far" can ever possibly discover how far it is possible for them to go. If we behave conservatively and do not take risks, we will always be slightly holding ourselves back. However, if we do not pause to consider the fear of failure, and do not allow ourselves to become shackled by the fear of doing too much or being too excessive, then we will be freed to fulfill our true potential.
A key word in this quotation, of course, is "risk." Eliot is suggesting that there is always an element of risk inherent in any real achievement. Only by welcoming this aspect of risk can we truly go above and beyond what we thought might be possible. If we want only to stay in our risk-free comfort zones, we will never establish what we are actually capable of doing.