Two Extremes of Metta
Repeating the words many times over might help you remember to do Metta when it is needed in a difficult situation. As a form of Metta it is very shallow, but nevertheless maybe beneficial. If you develop aversion when doing this best not do it at such times. When you get fed up just stop. Also the repetition may serve the basis for a change in attitude.

The most deeped way to do it is do Vipassana or Samatha Meditation. At some point you might get a pleasant feeling. When this happens bring ups the wish that others also experience this peace. This is more deeper form of Metta.

Objective of Metta is to:


develop and increase qualities Metta
break barriers thereby you consider some people of class of people more favorably than the others