The tip of my nose was square. Is it possible to recover it with a revisional rhinoplasty?

Thank you for you question.  How long ago was your rhinoplasty?  What work was done and how was the rest of your nose changed?  Is it more definition that you are looking for?  Many times, nasal tip definition is dependent on the width of the underlying cartilage and the thickness of the nasal skin.  I recommend posting photos so that we may further assist you.

Changing the shape of the nasal tip is certainly possible, but sometimes there are limitations.  A lot of these limitations depend on what techniques were used in your initial rhinoplasty.  Often times, the cartilage is trimmed, reshaped, and sutured to narrow a squared nasal tip.  Additional techniques can be used to create a more round appearance.  
There are several techniques to change the appearance of your nasal tip to create the shape you desire, but an in-person consultation would be the best way to assess your current shape, and your goals.  I recommend seeing a plastic surgeon who specializes in rhinoplasty and revision rhinoplasty.